Good freeware platformers:
Doukutsu Monogatari (Cave story) - with rpg elements (statistic etc)
Glace (not great but nice)
Pekka Kanna 2 - also very good for me.
Akuji the Demon - good, in Castlevania style
You can try also: Neko, Modarchive Story, Mario clones (Mario Forever is good), Sonic clones, Turrican clones (T2002, T4 Funeral), Mr Stumps Dentures and many many more.
Seiklus is also a little gem. Arcade adventure - Mario meets Another World
Fishead series looks fine but I didn't try this one.
There is a lot of good platformers on Snes. Sega Megadrive and PSX.
Greetings and sorry for my english.