Here's fourteen titles off the top of my head: Half-Life, Neverwinter Nights, Quake, Doom, Unreal, The Sims, Spore, Elder Scrolls, Civilization, Fallout 3, Bard's Tale, Lode Runner, Boulder Dash and Raid on Bungling Bay.
Some of those titles can be broken down into additional individual games (for example, The Sims was a significantly different game from The Sims 2, especially from a modding perspective) so this list could be expanded quite a bit. Some of them make modding a necessary part of their structure (Spore) and others wouldn't have garnered any attention at all if not for their mod tools (Raid on Bungling Bay) and others have just been outright owned by the mod community (Doom). And it's worth noting that even this short list of games represents a substantial portion of the entire market for video games on personal computers. The Sims and Half-Life alone have sold more games than the rest of the top ten list put together, and that includes World of Warcraft.
So, yes, modding is a significant factor in the success of "a lot" of great games on the PC.