Mount & Blade: warband comes to mind. I'd played the 1st mount & blade, but it felt meh...but an army of one Monkeh got me hooked on M&B:W for a while.
Most cheapo games I've bought were steam slaves of games I'd had for a long time in warez or on dusty CD/DVDs.
Mmmmm Warband, 400+ hours played, 6 euro paid.
Also Binding of Isaac. Best 5 bucks I've ever spent (well plus an additional 2x5 from normal gifting and 5x2,50 from gifting during the winter-sale and the 15 euro I paid for my Isaac Charm ).
Others include: Terraria, Killing Floor, Plants vs Zombies, Fat Princess (PSN) and I'll have to say TF2, since I wouldn't have bought that right away if it wasn't in the Orange Box already, yet it's the only game that still beats my Warband playtime.