The iCEnhancer mod is one for the graphics enthusiasts. The full engine mod adds several beautiful effects and higher resolution textures to the PC version of GTA IV. An overview of update 1.2.5, which is "almost done" according to mod creator Hayssam Keilany, can be seen in the video above.
Caveat: the mod apparently doesn't play nice with the latest versions of GTA IV. If you're unfamiliar with the mod, check out the above video, a kind of a mini-tour of everything it offers. And if you want to check out the mod for yourself, you can grab it here.