Xmonad - Xmonad is a Haskell based window manager for both Linux and Mac(sorry Windows users). It's highly configurable but for me it's lack of features and application bar crap is a real advantage. The application your currently working with takes up the entirety of the screen and you navigate to other applications using keyboard shortcuts. Using this and disabling notifications in your IM/Email/whatever clients forces you to work on the task in front of you. It's very pleasing to use after a while.
Focusatwill - I've been a big fan of using the Pomodoro technique and I use focusatwill as an extension to it. Effectively it's a timer that plays music that apparently helps keep you focused. Personally I've found it quite effective. It pushes for 100 minute stints of focused work before taking a break. As a coder I'm a big fan of this. 25 minute pomodoros got really distracting and were a pain to maintain after a while. 100 minutes lets me keep my mind on whatever level of the coding stack I'm currently on for longer periods. Even focusatwill states that it can take 20 minutes to accumulate enough to begin focus. Plus, if you're skeptical there's a free trial period.