Hell on Earth 3015 was a free, downloadable Pen-and-Paper RPG set in a not-so-distant-future. Development started in 2001 but it was not made public until 2004. The authors go by the Name Werner and Torben who created the website www.hoe3015.de - it is now offline but portions can be found at archive.org.
HoE 3015 was specifically designed to focus on (gun-)action and does not explain the world it is set in.
All checks are made with three d20 against a value set by the GM.
The game is distributed as an EXE file that "manages" the various game documents; essentially, it is a shell executable that works something like a table of contents, launching selected materials into a standard PDF viewer. The manager is not needed to utilize the included PDFs, however.