Emacs, some love it, some hate it, and once in a while I stumble on something I wish I'd found years ago. I thought I'd share some of the packages I use, partly so I don't forget!

• debian-el contains a major mode for editing sources.list, as well as an interface to apt for the truly emacs centric. Also has useful support for bug reporting and bug numbers...
• emacs-goodies-el lots of toys, and of particular interest and use to me, a major mode for apache configuration files and color-theme which is somewhat self explanatory
• php-mode, css-mode, html-helper-mode self explanatory, but I know a few people who have declared emacs to be useless because they didn't know they needed to install these packages.
• mmm-mode allows multiple major modes

all the package names mentioned above are as they appear in the Ubuntu apt repositories. There are countless other packages, mostly ending in -el or -mode that are handy, but the above are the ones I forget about most often.