I found a couple of cool fangames recently that I'd like to share...
Both are demos though.
Metroid II Remake
metroid2remake.blogspot.com It plays really smoothly, just close any other applications you may have running. If you've played any of the GBA Metroids (Fusion, Zero Mission) then you'll like this. It's a demo, so it's short.
Sonic eXtended Genesis(XG)
http://info.sonicretro.org/Sonic_XGI'm blown away by this. It's a fangame pretty much making it's own variation of the events after Sonic 3 and Knuckles. It seems like something Sega would've actually made if they were still in their right mind these days...
Anyways, post any fangames you've come across.