Thumper: Extremely fast, challenging and insane game, very unique and fun.
Half Minute hero 1 and 2: RPGs where you have a minute to finish each quest, and somehow it works.
Cave Story: Classic platformer, much easier than everyone claims until you get to the bonus stages but very fun.
Ben there, Dan That, Time Gentlemen Please, Randal's Monday, Dropsy and Space Quest Incinerations and Vohaul Strikes Back: Hilarious and creative adventure games with complex puzzles.
Super Mario Brothers X: A full game not to be confused with the ROMhack of a similar name, mixing a variety of fan-made Mario stage that also have elements of other Nintendo games.
Hotline Miami 1 and 2: Strange games with lots of challenge, fun, complex stories and creativity.
Rock of Ages 1 and 2: Strange games with Monty Python-style humor and Marble madness-style gameplay.
Zeno Clash 1 and 2: Surreal, subtle games reminiscent of Breakdown.