Some cool RPG systems that are free!
Some are also Pay-What-You-Want, if you wish to support the game writers and designers.
Probably my favorite version of D&D so far. Combines a lot of the best features of previous editions yet simplifies a lot. Now I feel that it’s less about min-maxing and more about character.
One of the quintessential generic RPGs using D100 (or percentiles). Pretty solid and dependable (this system is also known as Open Quest and was used in Call of Cthlulhu, RuneQuest and Delta Green).
An amazing way to look at roleplaying games. Really worth checking out.
Fate Accelerated is an even simpler version which is nevertheless really flexible and fun.