I recently discovered Garry's Mod for Source and it seems really cool. Then I saw some youtube videos of people playing around with GTA IV's gravity and friction settings (for the PC presumably). This got me wondering, if I get GTA IV for the PC (I've been playing in on 360 up to now), what mods are available? Do they only apply to multiplayer? If there are no mods, what settings can I tweak?
Graphics mods are popular - ENBSeries really makes GTA look good if you have the PC for it. There's a lot of model mods, which lets you add custom cars and helicopters and pedestrian models. They have a scripting language as well with .NET scripthooks, so you can modify more complex behaviors. In addition, they have physics tweaks (like you mentioned) that make the game a TON of fun. Mods work both in single-player and multiplayer (if a computer with the mods hosts and other computer that have the mods join.) I've been in a multiplayer map where cops were spawned in around a group and we were given tons of weapons by a admin-like player, who basically served as the "god" on that server. I've also played on one where ramps were spawned in the Liberty City Airport and a stunt movie was being made.
I hope that answers your question. Here's my favorite (and I believe, the definitive) mod resource for the GTA series...
Interesting timing, as I actually saw a couple reddit threads this morning discussing GTA IV graphics mods. As a disclaimer, I know very little about them, as I haven't had time to try them out myself. The mods are apparently from a set of mods known as the ENB series. When used in extremely powerful computers, they can produce screenshots like this. The screenshot was found in this thread and, after some digging, found this thread and some more screenshots. The website to download ENB mods is here. Installation instructions are on the same site.
As I previously mentioned, I have no personal experience with the installation or use of these mods. If you do try them out, I'd be very glad to know how it goes!