None of these are exactly OSX specific, but here's some stuff from my .bash_profile that I find useful:
Colored Prompt:
PS1="\[\e[0;31m\][\[\e[1;31m\]\u\[\e[0;34m\]@\h \[\e[32m\]\w\[\e[0;31m]\]\$\[\e[0m\] ";

SSH tab completion of hosts that exist in ~/.ssh/config: (found on MacOSXHints)
complete -o default -o nospace -W "$(/usr/bin/env ruby -ne 'puts $_.split(/[,\s]+/)[1..-1].reject{|host| host.match(/\*|\?/)} if $_.match(/^\s*Host\s+/);' < $HOME/.ssh/config)" scp sftp ssh
Highlighted grep:
alias grep="grep --color=auto"

Automagically dump your public ssh key to a host for future passwordless auth: (can probably easily tweaked to add said host to ~/.ssh/config)
ssh-setup() { cat ~/.ssh/id_rsa.pub | ssh $1 'cat - >> ~/.ssh/authorized_keys'; }
More OSX specific stuff that I've setup forces the machine to take a picture with the built-in iSight every time the machine's lid is open and dumps that image in a directory.
Requirements:
Create a directory somewhere to hold all your images.
Dump this into ~/.wakeup:
date=$(date +%y%m%d_%H_%M_%S).jpg;
/PATH/TO/isightcapture -w 640 -h 480 -n 3 -d -t jpg /PATH/TO/PICTURE/DUMP/$date > /dev/null
unset date
I've been capping a frame every time my MacBook wakes up for the past 3 1/2 years now, it's interesting to see everything compiled into a long video at a high framerate.
openfor everything: URLs, images, documents. I use it everyday. – olt Jul 6 '10 at 14:48open .opens it Finder. It's useful if you want to do something graphical. – ShreevatsaR Jul 26 '10 at 4:40