Firefox Extensions
If you're running Firefox and aren't using extensions, you really need to see what you're missing. If you're hesitant to plow through the list due to the sheer number of them, don't worry. Here's a list of the best extensions that I've found so far.
Fasterfox
Speeds up Firefox by changing a few internal settings. Useful if you usually have many tabs open.
FireFTP
Fast, decently-built FTP client. Not as nice or feature-rich as BulletProof, but free.
StumbleUpon
This is the biggest productivity killer ever. Tell it what categories you like, and it will take you to pages you would never have found on your own. When people ask where you found such cool pages, tell them you just happened to 'stumble upon them.'
CustomizeGoogle
Let's you remove ads and Google click-tracking from Google. Also allowed you to turn on Google Suggest, which pops down popular search queries as you type. Very useful.
ForecastFox
Adds the weather to your Firefox status bar.
DownThemAll
Speeds up downloads, allows you to pause and resume downloads.
Adblock Plus, Filterset.G
Blocks ads from known ad servers. I haven't seen a banner ad in months :)
Web Developer
One of the most amazing plugins ever. If you do web dev work, this allows you to edit css on the fly, view page elements in a myriad of ways, and countless other time savers.
There are many, many more, of course. They can all be accessed by clicking Tools->Extensions, and then clicking the Get More Extensions link at the bottom-right of the dialog box. Just one more nail in IE's coffin...