Lyle in Cube Sector: Extremely fun metroidvania-esque 8-bit platformer with a delightfully demented cast of enemies. Fairly short, but definitely worth a playthough if you like games like Cave Story or Metroid.
Darkside Adventures (and a bunch of other stuff): I haven't played any of the games on this page other than Darkside Adventures. Darkside Adventures is another fun, silly platformer with a bizarre setting and cast of characters. Not too long either, but there's a lot of stuff to collect, as well as a hidden bonus stage that can change the ending when beaten.
Legend of Princess and
Chalk, both made by the same dude who happens to be my fave indie game artist. Legend of Princess is a fast-paced one-stage action game spoofing Legend of Zelda, and Chalk is a fun little game where you drift about as some little girl while avoiding hazards and enemies by drawing lines to link things together or bounce away projectiles. Both good fun.
Also though it isn't freeware, Noitu Love 2 has my highest regards and is something you should buy if you love action games.
Eversion: A generic, cute, 8-bit platformer where you jump on things and solve puzzles to collect gems. However, underneath this exterior lies an unbelievably dark underbelly.