You've probably already beat the original One Unit Whole Blood and Blood 2: The Chosen (if not, what are you waiting for???), so here's a couple mods you might like trying.
[url=http://www.gog.com/en/gamecard/one_unit_whole_blood]One Unit Whole Blood and Blood 2: The Chosen, creations of Monolith Productions are two games that keep you in front of your computer for hours. We have some blood... I mean Blood enthusiasts here at GOG, so we asked one of them to prepare a present for all you Caleb fans. do you want to hear “I love the smell of napalm in the morning” or “I'm not dead...yet” again, as you kill numerous armies of zombies? Sure you do, that's why we found some cool Blood mods prepared by the awesome Blood community, played them and now it's time we share them with you.

Suggestion: It might help to tell people what game you originally *did* buy.

It is free, you cannot go too far wrong! :o)
It really does depend on your tastes.
If you like Dungeon crawls Stonekeep is a great choice.
If you like sort of crazy games and a sense of humor then Giants would be an obvious choice.
If you like turn based RPG games then Fallout, or Fallout 2 would be good.
Anyway, say what you like in a game and it will clearly narrow down your choices.