Software I use
2017 update: Interesting historical document! Well, interesting for me. Updates:
Mac: git (mercurial is better, but everyone uses git so I learnt git); macvim; Dungeon Crawl: Stone Soup (though I am now probably over crawlers); mpv for video; Firefox; Marsedit for blog posts. A variety of other things for special purposes (Omnigraffle, Bibdesk, NetNewsWire though it is being left to die apparently).
Linux: Debian; same software as Mac mutatis mutandis. I’ve switched to spamhaus’ RBL for antispam and that is working very well. Window manager is still ion3 \m/
Original, circa 2010
I use Mac OS X and Linux pretty regularly, and I've got lots of favourite bits of software on both systems. Figured I'd write about it. :)
Both platforms
mercurial: Pretty reasonable revision control software. Still feeling my way around it and am not entirely happy with the way it handles cherrypicking between branches. I used to use darcs, and I much prefer the way darcs operates, but it is unfortunately too buggy to recommend to anyone.
vim: Great, configurable text editor, usable anywhere, easy to script. It's worth taking the time to learn some of the tricks.
nethack: A lot of people like to talk about how this is "best game ever", but then they make it clear from the context that it's a joke. I don't get it.
Mac OS X
Mail.app: I used to use Thunderbird, but it was a bit tedious and not very Mac-friendly. Mail.app is of course incredibly Mac-friendly, but is missing some useful features. I use [Mail Act-On](http://www.indev.ca/MailActOn.html) to fill in some of the gaps. It works best on Leopard if you use the ctrl-key combinations rather than the backtick menu.
The usual suspects: I use Safari for browsing. It's much nicer than Firefox on the mac. It also crashes less often than Firefox nowadays. I use [SafariBlock](http://fsbsoftware.com/index.html) to block ads.
I use the iLife stuff – iPhoto (with the Flickr Export plugin) and iTunes. I keep GarageBand around just in case I feel like composing something one day. :)
Textmate: For when I'm bored with Vim. Happens less and less frequently now that MacVim is so great.
Marsedit: Blogging software that is easy to use, reasonably powerful, stable, and pleasant to look at. Costs money.
NetNewsWire: Very nice stable RSS reader -- now free.
Transmission: BitTorrent client. Free.
BibDesk: Great BiBTeX manager. Doesn't use its own file format – reads and writes .bib files. This single feature turns it from "curiosity" into "indispensable". Makes my life a lot easier. Free.
VideoLanClient: I find this better at playing video than MPlayer OS X. I also use Perian with Quicktime player. Neither is great. It's pretty funny (and sad) that Linux is better at playing videos than Macs.
Zoom: Infocom terp. Very nice and pretty. Part of Inform 7 now, which is itself pretty awesome.
Colloquy: Lovely IRC client.
Linux
Distro: Debian. It would take a lot to wean me from apt.
Serverside stuff: I use Apache 2 and Lighttpd (depending on where I am) for Web serving, and Postfix for email handling.
Spam handling: So far I've gone through SpamBayes, SpamAssassin, CRM114, and DSpam. I'm not happy with any of them, though currently I find that using SpamBayes with a million RBLs seems to be working out nicely. I don't use greylisting as I philosophically object to spam solutions that negatively impact real mail.
Bitlbee: This amazing bit of code is an instant-messenger-to-IRC gateway. It works by pretending to be an IRC server, and it's very lovely and means that I can use my favourite IRC client to talk to everyone.
X-Chat and irssi: Both great IRC clients, though I'm leaning towards irssi more now because I can run it on my server and leave it connected to bitlbee.
ion3: Window manager that doesn't get in my way. I keep a very austere GUI environment going on my Linux boxes and leave the pretty things to the Mac.
Firefox: Not much serious competition here. Konqueror is faster but much less compatible, less extensible, and crashier (though hopefully this will change if Konqueror gets a Webkit component)
mplayer: Very neat, full-featured movie player, finally going into Debian.
gqview: Picture viewer, few dependencies.