Perennially topical cult classic RPG Vampire: The Masquerade - Bloodlines has a thriving mod scene, and the one essential addition is the VTMB Unofficial Patch, a community-driven effort that's truly impressive in scope. Just this year it hit version 11.0 and, now, 11.1.
Bloodlines is has a fanatical following, and while the game was an ambitious and broken failure at launch back in 2004, the way the patch has polished it up since is downright impressive. Thanks to it, Bloodlines is revealed as a thorough and fascinating work, a game that puts you in the shoes of the engrossing and weird vampires of the World of Darkness. The GOG version of Bloodlines comes with the Basic version of the Unofficial Patch by default.
It's a cult enough classic that we're getting Vampire: The Masquerade - Bloodlines 2 at some point. Hopefully.
If you're not familiar with it, the VTMB Unofficial Patch, found on ModDB, incorporates fixes from the Basic Patch, which fixes scads of bugs, then layers on the optional Plus Patch (which isn't included with the GOG version), a mod that tweaks bits of the game that don't work well before restoring boatloads of unused content from the game files. That includes quests, levels, weapons, items, characters, even dialogue, graphics, models, and sounds from the original game. If you've never played, well, now's as good a time as any.