I use Auto Copy a lot on my work machine. I select text, and it automatically puts it on the clipboard.

Tab Mix Plus (link might not work today) and Adblock are a couple others that I wouldn't do without.

I find that some of the other extensions I used to have are now mostly built into the browser (eg, subscribing an RSS feed to Bloglines can be done from the browser now), so I'm running far fewer than I used to. I haven't done a lot of looking at the neat stuff that has come out in the past year.