Try out ZSH if you want to try something slightly different to Bash
  Package Manager: Homebrew - https://github.com/mxcl/homebrew
  Terminal/Emulator: iTerm2 - http://www.iterm2.com/
  Vim running in iTerm2 via Homebrew
  Chat Client: Adium - http://adium.im/
  MySQL DB GUI: Sequel Pro - http://www.sequelpro.com/
  Mail: Sparrow - http://sparrowmailapp.com/
  Git GUI: GitX (L) - http://gitx.laullon.com/
  Window "manager" : Shiftit - https://github.com/fikovnik/ShiftIt
  General productivity: I love QuickSilver, however it's been crashing
  quite frequently on me since I installed Lion, so I'm giving Alfred a try at the moment.

Note: all of the above are at least free (lite), if not completely free. I was thrown completely in the deep-end with terminal vim and just took to it as part of my "getting used to Mac" steps, which I think benefited me with respect to getting down with the nitty gritty of it and not complaining it was different to my Notepad++ experience prior on Windows.