I had almost 8GB of GMod addons before I wiped the game after it took over an hour just to change maps. (Amazing how messy things can get when one doesn't keep a check on what they're downloading, no? :O )
A few good ones, some of which are seemingly gone from the face of the Internet so I can't use them again:
-Spacebuild models - I seriously wish I could find the version of this I used to have ... all of the ones I can find are incredibly weak and incomplete, only containing a few generic pieces. The one I used to have had several sizes and styles of ship parts, all with tons of alternate skins and slight shape variations, cockpits as vehicles (so you could "drive" your ships), and much more ...
-Gravity Controller - Broken after version 2 - for me it now just spams the console with errors rather than doing anything useful. Version 1.2 was the one I used to have, and it worked great. Made anything (and any pieces attached) utterly weightless and could be toggled on/off. Ideal for use with the Spacebuild models mentioned above. I need to get this again as I haven't re-tried the old versions since wiping the game.
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Shuttle - The shuttle puts you in a headcrab canister (or whatever model happens to be associated with the version you have) and lets you fly it around. Much more interesting than noclipping to travel long distances.
-Quantum Storage Device - Too much fun for its own good.

Put anything inside and get it back out again (NPCs will die going in though). I like to use it for Ghostbusters-esque scenarios, unleashing vicious NPCs and trying to set the thing so it can capture them without getting killed myself. Be careful, as it hurts you while sucking in/spitting out things, and when damaged it malfunctions and sucks itself in, doing lots of damage in the process.
-Helicopter - The only decently usable air vehicle add-on I've ever come across. Kinda hard to fly at first, but lots of fun to crash.
-gm_trackmania map - A relatively simple map with a couple of racetracks. The paved one is hilarious with all of the trigger_pushes (I think that's what they are) on the jumps sending vehicles pretty much any direction they please (HL2 land-based vehicles were not designed for going airborne). :O Only the airboat can survive the whole course without flying to who knows where off the first jump...
If I think of more I'll post them, but those are my current favorites.