I'm probably more on the casual/old-school end - I don't enjoy spending long periods playing games, instead I prefer little 20-30 min sessions of something I can jump right into and get into without much complexities is perfect for me and good for breaks inbetween doing other stuff.
Quake (the original, hated the sequels)
Total Annihilation
Mortal Kombat (the old ones)
Super Mario Bros 1-3/Lost levels(smb2j)
Old PC games like Space Quest, Commander keen ect
I also enjoy some indie-games that are free to DL or play online such as Matt Thorson's Jumper series and other "masochistic" platformers. I really do love my 2D platformers.