Software I can't live without
RCDefaultApp Changes file extensions and associations on the fly
Locator GUI Find files anywhere on your system ( yes system files too! )
TextWrangler Edit and save files in various formats ( Awesome for administrators )
BatChmod Changes advanced permissions on the fly with a nice GUI interface
Carbon Copy Cloner Backup a system or restore a system ( Better than Apples software! )
Fugu A nice little SSH / SFTP / SCP client
Fetch A great little FTP client ( Must request Educational license to be free )

Other Cool Software
Language Remover Removes langage files on your system
iStumbler Monitor wireless connections ( including Bluetooth )
NetRestore Deploy MacOS images across a network
Netboot Across Subnets Umm, yeah what the name says
LoginWindow Manager Changes various options for Login
logGen Used for monitoring changes filesystem changes
RadMind The most advanced and powerful pieces of software I've seen for lab environments (and the most complicated!)
Passenger Simplify Mass accounts imports! ( * Warning * not free software!!! )
CroniX A GUI for Cron jobs. A.k.a. performing certain tasks at certain times.
Lingon Similar to Cron, but uses a GUI of LaunchD
Ldapper Browse Ldap databases