So, Garry's Mod's been a lot of fun lately; usually the game outlives its novelty after a day or two, but playing with a number of friends and using cool maps really makes a hell of a difference. The amount of shit you can do with it when you factor mods into the equation extends its entertainment value by orders of magnitude.
Speaking of which, I think it's time for a new post that has updated information about all the mods and maps we've been using lately. Some of these addons require SVN to download, so get Tortoise SVN. Once installed, right-click your desktop, choose SVN Checkout from the list, and enter the URL provided in the entry. You'll connect to the SVN server and download the latest copy of all the files. When you're done, just move it to garrysmod/garrysmod/addons. You can update your copy at any time by right-clicking the folder and choosing SVN Update.
And now, the list (last updated 4/14/09):
Zombie Forest (zm_zombieforest_b7): This is an incredible map. It's a very large, dark, rainy, spooky forest full of logs and boards from broken shacks and gravestones and the like. The idea is to play without spawning anything, just use the available resources to build a makeshift bunker to protect you when the zombies come. They can randomly appear at any time between 24 and 30 minutes after the start of the map, and when you're dead, you're out of the game (unless you noclip and cheat, but that's cheating!), making it very important to construct a solid defense. Phenomenal design, highly recommended.
gm_wireconstruct_rc: Mentioned below, this is an awesome map for general building. Huge map with train tracks, water, missile-building platforms, all manner of stuff in a clean and friendly environment.
rp_zombiesurvival: Fun map with constantly-spawning zombies in a town full of a lot of interesting buildings. Wears thin after a while but fun for a time!
WireMod - Advanced stuff for GMod, from useful (creating controllable winches) to advanced (making a roaming rocket-firing auto-targeting turret out of blocks) to esoteric (in-game routers sending packets and shit to each other). Necessary for GCombat and most truly useful stuff, and it comes with Advanced Duplicator, which lets you save your work and upload it to any server. This also requires SVN; on this one, username/password is anonsvn/anonsvn.
GCombat08 and GCombat Extended: Destructible environment stuff, tons of cool weapons from SAMs to nebelwefers to laser cannons. Great fun, lots of fire and doom. Requires SVN.
Simple Weather Mod V1: Client-side mod that adds a bunch of weather effects. Pretty spiffy, the "storm" setting makes any map instantly creepy. (OPTIONAL)
Dragon Snpc: Totally optional, but totally bitchin' dragon from Might and Magic. (OPTIONAL)