I'm a programmer to be prepared for some pretty weird/geeky picks...
MacVim - as someone who spends 90% of their day in a text editor once you learn vim or emacs properly... you never go back. It's a steep learning curve but once you've got the basics it's the gift that keeps on giving. Editing using sublime now feels incredibly clunky and slow
Sequel Pro - a pretty nice mac GUI for MySQL databases. Running under Alfred it also has a rather amusing alias where you can type "pancakes" and it'll come up. Check out the icon for why this is funny to me.
Jq - a command line tool that lets you query JSON. Oh my goodness me - it's just the best.
Zsh / terminal - once you really get the power of the terminal it's like a great editor - you never go back. It's so impenetrable and it drives me mad how few people explain the power of it well. I used to hate it with a passion. But once you experience the insane power it becomes unbelievably addictive.
Ruby - a truly lovely programming language. Once again, I don't think there's anyone I know who teaches Ruby or programming in a way I like. Once again, once you get comfortable with it and programming... it changes everything!
Alfred - I hate using the mouse. Hate hate hate it. So clumsy and slow. Alfred let's me launch apps, send emails and so much more.
SizeUp - see my line about using the mouse - once of my favourite bits of software ever, size up ditches clumsy awkward dragging windows on the Mac and resizing them in favour of quick, precise no nonsense keyboard shortcuts for full size, half screen and next monitor Windows. Freaking amazing.
Git - once again, I didn't really understand it for the longest time. It takes some investment but once you start using the command line and finally get what's going on its so useful. I couldn't survive without it now.
GitHub - I signed up very shortly after GitHub was started. Over the years it's matured into an incredible bit of software for sharing and collaborating on code and a lot more. Simply indispensable.
If anyone would like to know any more about these tools just ask!