It has been a almost 3 months I switched to Mac using it full time for development and day to day usage. It has been an awesome experience.
During this period, I have installed and uninstalled many tools.
Below are some of the best productivity tools that I'm currently using which surely make the development experience a memorable one.

iTerm2  - the best Terminal - must have
Producteev - The Ultimate Free Task Management Solution. Check it out. It even sends reminders of the late tasks, very intuitive to use
Alfred - makes searching the Mac a breeze. A good alternative for spotlight, lots of features for customization
Sophos  Free Antivirus for Mac. Better to be safe than sorry
Magicprefs  - MagicPrefs is a free application for OS X which aims to improve the functionality and configuration options of the Apple Magic Mouse, Magic Trackpad and the MacBook glass trackpad
TextMate - the missing text editor for Mac
TextWrangler - another awesome free text editor, only limitation - no tab support. I used to love notepad++ on Windows. After downloading TextMate, no looking back.
DTerm - is a handy command line tool - A command line anywhere and everywhere. Use it to believe it
EasyFind - Worthy replacement for in-built Spotlight. Searching a file is many many times faster than Spotlight. Lots of options, it compliments with Alfred quicklauncher

..more later