Updated: I keep a more current list on the wiki.

Now that Mark Pilgrim has switched to Ubuntu he's updated his essential software list. Here's mine:

1. Ubuntu, Linux for human beings.
2. Vim + Markdown Vim Mode + bufexplorer + svncommand, for people who don't like taking their fingers off of home row.
3. Nautilus File Manager. Put Nautilus in spatial, tree-view mode and create some connections to remote servers you access regularly and you won't know how you lived without it.
4. Firefox + Adblock + Firebug + Greasemonkey + Gmail Smart-Delete Button + mplayerplug-in. I try to keep things simple.
5. Gmail. I still haven't found any other web or desktop mail reader that has one-key archiving plus fast, indexed search, which is the killer Gmail combination.
6. Google Reader. I was using feeds.reddit.com but it recently lost the essential feature of hiding read items.
7. Backpack, I'm still keeping my life in here.
8. GAIM. GAIM has plugins? Who knew?
9. gThumb. Who needs camera software? I use a card reader and organize photos in good old-fashioned folders before uploading them to Flickr.
10. Rhythmbox, mostly just for downloading podcasts.
11. Rhapsody.com, all-you-can-eat, Linux-compatible, DRM-free (I think) music streaming.
12. Beagle, fast, indexed desktop search.
13. GNUCash. It's old and ugly but rock-solid and I have years of data in it.
14. GNU Screen. Run console apps detached! Do everything in a single terminal window! Impress your friends and strike fear into the hearts of your enemies!
15. OpenSSH. Kind of goes without saying.
16. rsync + scripts based on Mike Rubel's for backups.