vim A sophisticated text editor with no GUI – everything is text-based, and all commands are issued from the keyboard. Frightening at first, and then so ridiculously efficient that nothing else makes sense.
Firefox Great in and of itself, and the ability to add extensions makes it priceless.
Firebug One of said Firefox extensions – as Marcus said recently, it’s hard to imagine how Web 2.0 would have happened without it. The Inspect function, the console, the Net view … without it I’d be utterly lost.
Filezilla I booted up an old computer of mine and found an FTP client I think I had to either pay for or steal, and it wasn’t half as good as this. I wish it had bookmarks for moving around servers easily … alas.
Pidgin All my instant messaging personas in one application, with pounces, fine-grained status control and logging. Plus: file transfers seem to work better now. Woo.
cygwin In an alternate universe, I get to use a Mac. Until that alternate reality arrives, cygwin gives me the power of the terminal in Windows. These days, I always have at least one cygwin window open most of the time, because sometimes, things are just easier on the command line.