I have some stuff mentioned like AdBlock and LastPass (which is my only must-have extension). Others I haven't seen mentioned:

AutoHD for Youtube - supposedly forces the highest quality for Youtube videos, though I'm not sure if it works

Minimalist for GMail - great customization for removing elements from GMail

Google Voice - easily make calls from my computer and check my inbox instead of going to the voice website

Google Calendar - just a nice quick way to bring up my Calendar

Context Menu Search - right-clicking on highlighted text lets me search predefined search engines (Wikipedia, IMDb, etc)