Below are the most useful commands (or at least those I found most helpful) when I switched to Ubuntu, to clarify, these commands are not Ubuntu specific, they are Linux commands. I’ve not included some of the basic commands like cd, rm, mv, ls, etc… Play with your terminal, that’s a great way to spend a Friday afternoon! Hee hee hee… (Yes, I’m a geek!) 
These commands are in no particular order, I just find they are the more helpful (for me and a few of the “converts” near me).
sudo
Execute each command as root, which is safer than actually switching into root via “su”.
ifconfig
Displays the network card connection/configuration.
sudo apt-get install PackageName
Will install the whatever “PackageName” I typed and the dependancy files.
sudo shutdown -h now
Shuts down my Ubuntu and powers it off right away.
&&
Command to complete more that one action. For example:
sudo make && make install
Which runs the “make” script and then installs the “made” package.
ping 10.0.1.1
Sends test packets to whatever IP you type in, to help troubleshoot network connections.
apt-get moo
Just to look smart! Try it!
cat /etc/issue
Displays the current Ubuntu version.
wget http://syserr.com/stuff/madwifi-cvs-20051025.tar.gz
Will download the file from any URL you enter after “wget”
tar -zxfv madwifi-cvs-20051025.tar.gz
Will uncompress the downloaded file
z means “Gunzip(uncompress) it before extracting, used on file ending in .tar.gz or .tgz”
x means “Extract the contents of the TAR file”
f means “Filename to follow”
v means “Verbose - display contents as it is tarring or extracting”
chmod +x install.sh
Command that says “Make the script file install.sh executable”. I sometimes run into this after using wget and trying to run installer scripts.
sudo apt-get autoclean
Removes partial packages.
sudo cp /home/roger/backups/bk03072008.tar.gz /var/www/downloads
Copies a file from one directory to another directory. (in this case the file bk03072008.tar.gz was copied to the /var/www/downloads directory).
If you don’t want to copy but move the file, change cp to mv.
find . -name “*.odt”
When looking for OpenOffice documents (in the current directory and all it’s subdirectories) or find ./roger -name network_notes.odt when I know the exact name of the file.
..
Move up one level in the directory structure.
iwlist scan
Dislay wireless networks that are in range.
mkisofs -V LABEL -r dir | gzip > backups.iso.gz
Makes a CD image of the backup directory contents
cdrecord -v dev=/dev/cdrom blank=fast
Quick command to erase a CDRW
history -c
In a multiuser environment, it is sometimes not good to leave history. Of course you can just “exit” to leave the terminal.
arch
Displays the processor architecture so I know if I should download i386, i686 based application pakages.
whereis something
Displays where “something” is stored.
There are 100’s more. But these are the ones I tend to use more often. As usual, I hope this helps others! 
And… one for the road…
If you want to create a file called “me”, just “touch me”
There are 3 comment(s) added so far...
“sudo shutdown -h now” can be shortened to simply “sudo halt” Also, I think you need a “cd” before that “..”
The z for .tar.gz’s and j for .tar.bz2’s are no longer necessary. Tar just kinda figures it out and does it automatically. “tar xf ….” works just fine on both.
&& isn’t exactly what you think it is either. To just list commands, use ; What && does is says “IF the command on the left completes successfully, THEN do the command on the right.” You can use || to say “IF the command on the left fails, THEN do the command on the right”
Oh, another way to get the distro version, which I’m pretty sure works cross-distro (/etc/issue isn’t on Fedora systems, I don’t think), is “lsb_release -a”
@Mackenzie - Thanks for the input! If make fails for some reason we would not want to && make install
Cheers!