So stop your cheap comment, 'Cause we know what we feel...
1- Sealed Earth
Perhaps the earliest historical "potions" used were medicinal earths. Usually these were types of mineralic clays, including kaolin and bentonite. Sometimes they were used as found, other times they were mixed with iron hydroxides ("green rusts") to form "ochres", basic pigments. These medicinal earths were typically used topically though they frequently were taken internally by geophagy ("earth-eating"). In ancient Egypt, medicinal clays were used as anti-inflammatories, consumed for intestinal problems, and also used in mummification. In ancient Greece, Lemnian clay (mined on Lemnos) is used topically and also mixed with vinegars and wines for internal use against poisoning and other internal complaints. This practice was common enough that Lemnian clay was commercially distributed as tablets (small cakes) with various seals pressed into the surface before drying. These seals testified to the tablets' purity and authenticity. Because of this practice, the tablets came to be known as terra sigillata, or "sealed earth". Numerous clays from other sources also were used for various specific complaints - primarily topical, but sometimes internal. Medicinal clays were widely valued during Medieval times - especially in Muslim Spain. During the Renaissance a naturalist noted that true "sealed earth" was harvested only from Lemnos, and was harvested only once per year in a special ceremony. Medicinal earths and clays are today still frequently found in a variety of products - everything from complexion creams, clay masks, mud baths, and even supplemental powders. Today clays primarily are used topically for skin conditioning. A modern product - QuikClot Combat Gauze - is traditional gauze infused clay-derived microparticles. Kaopectate, Rheaban, and Diar-Aid all are clay-containing products, though the FDA has stated they are not effective.
Adapting sealed earth to fantasy genre gaming promises to be both simple and intriguing. The most obvious analogues are as healing or poison neutralizing potions. While taking a clay tablet is probably more difficult than drinking a potion, a clay tablet also is much more durable than a glass vial. They may also be smaller and lighter (depending on implementation). They also could be used as a distinguished form of "clerical" magic in favor of a liquid "alchemical" magic. Depending on the economic implementation, sealed earths may be much easier to procure/fabricate than traditional potions - and of course the "special locales" that yield efficacious magical clays would be strongly guarded. It would be easy to see how a certain e.g. monastery would be fantastically wealthy if it was able to harvest, dry, and sell mud.
2- Dragon... sweat?
The traditional potion (technically, a "spagyric" – how’d Gary Gygax miss that one?) is manufactured by an alchemist using bizarre forms of extraction and distillation. Everybody knows alchemists buy various weird body parts and rare plants, and then cook them up into potions. The stranger and more obscure the ingredients, the more powerful (and more expensive) the resulting potion. But what about potions that are not made that way? Most fantasy games have highly magical creatures. It's only common sense to assume that various parts of these magical creatures can be converted into potions by an alchemical expert - but why couldn't other parts of these magical creatures have magical effects without any alchemical intermediation? There are some monsters that have dangerous blood - characters take acid or poison damage during combat - why wouldn't there be other monsters that have beneficial blood? Or sweat? Or tears?
3- Tippling for Health
Tinctures are alcoholic extracts of plant or animal materials. Tinctures usually are about 50 - 100 proof (25% - 50% alcohol). Almost all modern tinctures – a great variety - are quite concentrated and the actual "dose" is small. This has not always been the case, however. Older extraction processes were less efficient. In most games, "potions" are deemed to be about one ounce in size, more or less. While this standardization works wonders for encumbrance, it's not particularly interesting. It might be entertaining to have a potion of neutralize poison to be roughly the size of a pint or quart of hard alcohol. Yes, it will neutralize the spider poison, but it confers another type of poisoning, so to speak. You won't die, but you'll be incapacitated soon enough. And in the morning you'll wish you'd died. If you want the one-ounce size, why, that's nearly twenty times as expensive...
4- Anybody remember Keoghtum's Ointment?
By far the most common type of potion in most games are the drinking type. Because of Gygax's beautiful vocabulary, everybody knows that you "quaff" a potion. And certainly for today's healthcare system that makes a lot of sense - the really powerful stuff almost always is taken internally. Magic need not follow science, however. Keoghtum's Ointment was an excellent example - if you ate it, it did one thing - if you smeared it on topically, it did something else. To me, topical application for healing wounds always made much, much more sense than quaffing. Plus there's the added advantage of being able to topically treat a "comatose" friend instead of somehow trying to dribble a liquid down their comatose throat. Even neutralize poison effects could be topical on the site of the poisoning. I think the topical application process is given short shrift in most games. But in Pokémon a variety of "potions" are used - they are shown as being more like sprays that are applied topically than as potions that are imbibed... err... quaffed.
5- Wait... I think I have a potion of flying...
I'm sure everybody has played a character that picked up a potion around about first level, tossed it into the non-magical backpack of infinite space, and then forgot about it until some desperate situation around about tenth level. Faced with the rapidly occurring TPK, it was nice to drag out that potion of flying and escape to tell the tale. But, maybe not. Most modern medicines have expiration dates. Even though they are chemically pure, professionally formulated, and stabilized - they still lose efficacy and eventually aren't worth anything. Obviously some take a lot longer to expire than others, but the point is that besides maybe water, every liquid eventually goes off. From the week or so of milk to the centuries of wine, somewhere in there is a spoiling process. Why not potions? Maybe they last a month, maybe a year, maybe twenty years. Maybe it's a known process, with manufacturing dates noted on the bottle - maybe it's a random process that happens sometime between the first and third year. Maybe a healing potion loses 10% power for every month it spends in the bottle. Who's to say that a decades only neutralize poison not only doesn't neutralize poison but has itself turned poisonous?
6- Potion Miscibility
Every time you pick up a prescription from a pharmacy, the pharmacy's dispensing computer system is going to perform a drug-allergy and a drug-drug interaction check based on whatever allergies and other prescriptions you have noted in the system. If an adverse reaction is probable or possible, the dispensing system will alert the pharmacist to check the order. This is done because drug-drug interactions are common, serious, and often dangerous. Like many intriguing but difficult rules in gaming, potion-potion reactions have a long history, stretching clear back to Advanced Dungeons & Dragons (1st Edition). Gygax provided the "Potion Miscibility Table" on p. 119 of the Dungeon Masters Guide (AD&D 1e). Whenever your character drank a potion while under the effects of another potion, you had to consult the table. It was sort of like Russian Roulette, although back in the old school days we sometimes did it just for kicks - because one time in a hundred one of the potions became permanent, and usually you could survive the 99 other times (especially when you were around eighth or ninth level). I'll just say it - I love the potion miscibility table. I think it adds a huge dimension of uncertainty and just funny tension to the game. It's been "reinvented" many times over and lots of the spin-off miscibility tables and rules also are great. I really do miss that loosy-goosy strangeness of Gygax's game - completely lost in e.g. 3rd Ed where everything was a defined effect. I guess you can't drink your potion and have it, too...