Fuzion is a generic role-playing system published from a collaboration of two RPG publishers (R. Talsorian Games and Hero Games). It is a combination of two previous systems (the HERO system and the Interlock System) and is a very flexible and open system. There are two published versions of Fuzion: Instant Fuzion and Total Fuzion. Instant Fuzion is the more basic version, Total Fuzion is the fully detailed version.
Fuzion was one of the earliest Systems to be published free on-line to be used by GMs. It also was licensed to other publishers but not under the open licenses that came later. Licensed games use the Fuzion Powered logo. Fuzion is popular with the anime themed RPGs.
Fuzion is designed with Plug-Ins, Keys, Switches and Dials. These allow the GM to choose which parts of the Fuzion rules they want in their campaign without creating imbalance.
A Plug-In allows the GM to use rules from other systems such as the parent systems. These Plug-Ins are simpler, condensed versions of the rules sets from other RPGs.
Switches allow the GM to pick which parts of the rules are active in the game they run. For example, a game with Mecha Rules switched off will not have Mechas.
Dials allow the GM to set the level of a particular rule to suit the campaign.
Keys are novel or interesting ways to use rules and are intended to help the GM recognise important elements in the rules.