The addons that really do "add on" to GMod; that improve it as a sandbox, toolset, and game - the ones I'd consider "essential", are:
Advanced Duplicator 2
Wiremod
- The unofficial Wiremod extras pack (UWSVN)
- Fin
- Easy Precision
- Weight
- Buoyancy
- Unbreakable
- Stacker
The less essential but not on the list (due to being too specific or too akward for multiplayer):
- PewPew
- Gravity Hull (Local Physics)
- Advanced Teleporter
- Make Spherical
- Breakable Props
- Door
Google them, make sure to go for the one that looks the most legit, recently updated, and widely used - generally, if a project has an SVN, then use that as the source rather than downloading a static archive file.