A few tools that are invaluable already, a few that are becoming so, and a few that I think will in the future. This is the junk (Anyone watch that Extreme Makeover: Home Edition show?). Some are more well-known than others.
meld: graphical diff/merge
wiggle: applying patches with conflicting changes
quilt: patch manager
herdstat: Gentoo-specific querying tool for herds, developers, categories and packages
splat: Gentoo-specific portage log analyzer
esearch: Gentoo-specific caching package search. Way faster than portage.
cpu: CLI LDAP user management. May try switching to diradm soon, since its by one of our Gentoo devs, robbat2.
logrotate: keep log size manageable
sudo: selectively let people run commands as root
superadduser: for people who don’t like reading the useradd man page
nano: a surprisingly featureful editor
screen: detach running sessions
detox: cut weird characters out of filenames
strace: useful in debugging
ccache: speed up repeated compilations
keychain: SSH agent manager — don’t type the password every time

I always enjoy reading posts about usability design, because it’s something that way too few people in the OSS world know anything about. “Designed by hackers, for hackers” as a motto seems to say “Figure it out yourself — I ain’t into that intuition thang or writin’ no manual.”

I’m definitely in agreement with Bryan on that — design trumps consistency when they’re in conflict. Designers are often afraid to make choices on what’s best for the user, so you end up with these absolutely flooded preferences dialogs (Firefox, anyone?).

This post reminded me of a useful portal for collaboration between OSS developers and usability experts.

While I’m on the subject, I figured I’d mention Joel Spolsky’s book on user interface design for programmers again.