Gametap's free, ad-supported service is now here. 31 free games that are changed every week. They also have games for sale if you're so inclined. And of course, if you pay for Gametap (starting at $10/month and gets cheaper if you buy a longer subscription), then you've got a veritable buffet of gaming goodness - 893 games currently available, and more on the way.
No excuse now! Go and have fun! If you have a PS2 pad, go get a PS2 to USB converter. If you have a 360 pad, get the Windows drivers for it! Hurry up! You should be playing Metal Slug!