* QuickSilver (Launcher app), http://www.blacktree.com/
* TotalFinder (Tabbed finder), http://totalfinder.binaryage.com/
* iTerm, http://iterm.sourceforge.net/
* CSSEdit 2, http://macrabbit.com/cssedit/
* TextMate, http://macromates.com/
* AquaMacs, http://aquamacs.org/
* Tower (awesome git client), http://www.git-tower.com/
* Araxis Merge (crossplatform merge), http://www.araxis.com/merge_mac/index.html
* Aqua Data Studio (sql editor), http://www.aquafold.com/
* Adium (instant messenger), http://adium.im/
* DropBox
* VLC (Media player), http://www.videolan.org/vlc/
* Perian (plays xvid/divx avi's), http://www.perian.org/
* ExpandDrive (mount sftp, ftp, etc. as volumes), http://www.expandrive.com/mac
* 1Password (password manager), http://agilewebsolutions.com/onepassword
* VoodooPad (desktop wiki), http://flyingmeat.com/voodoopad/
* Unison (usenet browser/downloader), http://www.panic.com/unison/
* Scrivener (writing app), http://www.literatureandlatte.com/scrivener.php
* MacPorts (package installer), http://www.macports.org/
It's worth installing the developer tools from Apple. Also, you'll probably want to install MacPorts, which is a package management system similar to what you are used to on Linux.