I've recently been somewhat addicted to independently developed games. They're usually much shorter than the multi-million dollar productions that you see in shops.
A good example is Knytt (download here: http://nifflas.ni2.se/ )
It's a charming little metroidvania. It plays well, isn't frustrating, and you'll finish it in a few hours.
For something a little harder, check out La Mulana ( http://agtp.romhack.net/project.php?id=lamulana )
Retro graphics, awesome music, and a huge world to explore (and frequently die a horrible death in).
More stuff here: http://www.tigsource.com/
Enjoy.
There's one I liked a lot, Last scenario
http://www.google.com.br/url?sa=U&start=1&q=http://www.baixaki.com.br/download/last-scenario.htm&ei=iqSySbjyJce_tgeonY28Bw&sig2=hhICI-elhehIrD1vgiEomw&usg=AFQjCNEIN9Jo09yLS965wWv2sLfSnqRZ9w
a well-written RPG with a good gameplay. Too bad it is Windows-only
5 Days A Stranger (http://www.fullyramblomatic.com/5days)
7 Days A Skeptic (http://www.fullyramblomatic.com/7days)
Trilby's Notes (http://www.fullyramblomatic.com/notes)
6 Days A Sacrifices (http://www.fullyramblomatic.com/6day)
A four game series known collectively as the Chzo Mythos, a set of classic-themed 2D adventure games in the design of text parser and point n' click. Made by Yahtzee before he became ever so popular as a game reviewer - or at least, I found them before he became ever so popular, heh. Horror, gore, supernatural themes, and a convoluted overarching plot, all in precious 2D.
You should play them. *nod*