Your Garry's Mod game has so much wastet potential, if it doesn't use addons. Addons used to be shared by direct links only, which had to be downloaded and installed in the \Steam\SteamApps\common\GarrysMod\garrysmod\addons\ folder.
Though nowadays addon developers want to update their addons more or less frequently so they started to use subversion (SVN) first, which lets users update their addons using a simple program. The easiest tool to use SVN is probably TortoiseSVN.
Since the GMod 13 update you can easily upload and install addons using the built-in steam workshop. For clients and servers, the updates are completely automatic.
Armored Combat Framework
||Adds weaponry with simulated ballistics, a damage system, engines, differentials and transmissions.||https://github.com/nrlulz/ACF/branches/master|
Advanced Duplicator 2
||Duplicates contraptions and saves them to your hard disk, supports old Advanced Duplicator files.||http://advdupe2.googlecode.com/svn/trunk/AdvDupe2|
Generic Default Combat Weapons
||A set of weapons with nice models and realistic bullet physics.||http://generic-defaults-combat.googlecode.com/svn/gdcw|
||An ingame tool to do hologram modeling, which can be compiled to expression 2 files.||http://holopad.googlecode.com/svn/trunk|
||A large set of highly detailed cars from various brands.||http://gmod-dl.googlecode.com/svn/trunk/TDMCarpack|
||The essential addon to make proper I/O systems and many other linkages, where you normally had to use your numpad.||
Unofficial Wiremod SVN
Wiremod tools, which are not supported anymore, but useful anyways.
||Duplicates contraptions and saves them on your hard disk or on the server; this tool is the original wiremod duplicator tool.|
||Allows non-wooden props to be able to float, useful for boats.|
Climb SWEP 2
||Allows you to do parcours like jumping (first person only), including wallruns, grabbing ledges, etc.|
||A tool to apply/copy colour and material from/to one prop at the same time.|
Easy Bodygroup Tool
||Some props are able to use different skins or body groups; this tool lets you change them with a simple right-click.|
||The fin tool allows you to use the dynamics of props to let them be affected by their speed and their resistance angle; this is useful for spoilers, boat rudders, plane wings or even propellers.|
||Allows you to make any prop a lua-coded sphere; useful for wheels, which would, under normal GMod physics laws, jump or sink into the ground.|
||Allows you to change the model of a prop whilst maintaining the colour, material and angles.|
Multi-Parent / Unparent Tool
||Allows you to (almost) laglessly attach props to another; parented props don't react to the world or other props.|
No Collide All Multi
||Can disable collisions with other physics props for many props at once.|
PHX props missing materials
||The PHX model pack has not alway been integrated in the regular Gmod, but when Garry implemented it, he didn't import all textures properly; this addon fixes any texture errors.|
||This is useful, when you want to build something, which should tear apart realistically; the tool automatically creates force-welds to the adjacent props.|
||Restores important and useful tools of the previous GMod version.|
||Adds a grid to which you can snap to with your toolgun while building; useful for accurate building.|
||The essential and optimised prop pack for every GMod builder.|
||Allows you to easily stack multiple props onto or next to eachother.|
||Allows you to sit on anything, whether it's moving or not.|
||Can visually clip props so that you will only see the unclipped part of it.|
||Can set a custom weight to a prop and most entities.|
||A pack of realistic clean textures, useful for building.|
The essential addon to make proper I/O systems and many other linkages, where you normally had to use your numpad.
Warning: This addon also exists as SVN download. Usually the SVN version is the newer version out there.
||Allows you to accurately place constraints, works best in use with SmartSnap.|
|Axis / Ballsocket Centre||Allows you to simply create Axis and Ballsocket constraints to the center of props, very useful if you want a prop to spin above/below another prop.||https://dl.dropbox.com/u/7934790/gmod/stool_axisbscentre.zip|
|Physgun Buildmode||Useful if you only use a touchpad mouse, but also for regular players; this allows you to carefully move props with the physgun whilst keeping their orientation and such.||https://dl.dropbox.com/u/7934790/gmod/physgun_buildmode.zip|
||Garry didn't implement the original PHX spawnlists, when he added PHX to the official game; this is a rough fix (one list is still missing)||http://1drv.ms/1eOx95N|
|Precision Alignment||The non+ultra addon for accurately placing constraints.||https://dl.dropbox.com/u/7934790/gmod/stool_precisionalign.zip|
||The predecessor of Make Spherical, do not use this unless you want it to try out old duplications that still use Wheel2.||http://1drv.ms/1eOxG7D|