Chrome doesn’t really have any cool add-ons yet and a couple of Firefox ones that were ported over don’t work right.
Firefox though, there are a few add-ons that I love.
Toolbar buttons – Adds a lot of (optional) buttons that base Firefox lacks. It makes customizing the menubar so much better.
Hyperwords – It does so much that you really have to visit their site to find out how cool it really is. Unfortunately, the Chrome version got screwed like a $2 whore in the transition. It’s a little funky to configure at first, but You have to check it out! I don’t know how I surfed without it.
Leet Key – The “leet” function is a bit lame, but I often use it to translate to/from hex, binary, or ROT13, though it has a few other minor uses
Fireshot – Great for screenshots. Has a few nifty features.
Image Zoom – Does what it says
Video DownloadHelper – Best tool of it’s kind that I’ve tried so far.
AdBlock Plus – If you know what you are doing, it has other functionality as well.
@jaytkay If you had HyperWords, you wouldn’t need either of those two lookup extensions. You also wouldn’t need certain toolbars, translator utilities, currency converters, and a lot of other things.