Chipzel has already gotten a mention, but I have to say her stuff is fantastic, especially Chipped of the Necrodancer and Super Hexagon.

Developer Konjak (Joakim Sandberg) did good work on the Noitu Love 2: Devolution soundtrack, which is a free download (zip file) from his website and is very chiptune. The actual link on the website's main page has an errant slash at the end that has to be deleted for the download to work. Some of the album can be found on YouTube if you want a preview.

Another banger chiptune to go for comes from Zircon and the work he did for the Demon Truck Soundtrack, especially The Devil's Mudflap. It's 16-bit Genesis goodness, and the game is pretty fun! Zircon's done stuff for several games and is generally an all-around musical badass.

Herbert Weixelbaum's Tanzmusik on the album 8-Bit Operators: The Music of Kraftwerk is a total earworm. The album is full of good chiptunes, but that one's my personal favorite.

You might also enjoy Adventure by Adventure. Very upbeat and energetic stuff.

Lastly, I have to give a shoutout to Zweihänder. All his stuff is pretty good, and much of it game-inspired.

Enjoy!