Steam from Valve, the creators of Half Life, Portal, Left 4 Dead, Team Fortress and more games, is the best distribution service for PC gaming available. They have amazing sales and recently concluded their annual Steam Summer Sale which sees games go on sale for anywhere from 50 to 80% off of the listed price.
They also frequently have 75% off sales during the week. This past weekend they offered the original Borderlands as a free to play weekend, this was the full game, from friday night to sunday early afternoon. They also had the full game on sale for $4.99, and the Game of the Year Edition (with all 4 pieces of DLC) for $7.49 (it's about $30 normally at Amazon). It was a no-brainer. After trying it out for several sessions I purchased it.
Steam is amazing for getting great deals on games, many people have commented that they don't ever pirate games anymore because Steam has games for such a good price. I think that I have spent around $25 on Steam and have purchased Plants vs. Zombies: GOTY, Bunch of Heroes, Portal 2, Defense Grid: The Awakening, Borderlands: GOTY, and also have access to free to play Super Monday Night Fights and Team Fortress 2.
Steam is your best option for gaming on the PC, try it today!