Already a lot of great software mentioned, so I'll supplement a few

Alfred – http://www.alfredapp.com/ Don't put it off as "just another application launcher". It DOES launch apps, and that rather quickly and elegantly. But that's also what Spotlight and Quicksilver (free, but afaik out of active development) do really well. Alfred's strength lie in it's versatility. Need a Text Expander? Can do! Want a Multi-Clipboard? Yep, got you covered! Also, if you know just the slightest bit of AppleScript, you can create really powerful shortcuts tailored to your specific workflow. Alfred is super versatile and worth a second look.

Fantastical – http://flexibits.com/fantastical Intuitive Calendar. (Of course, you can also create new entries from within Alfred...)

Sublime Edit 2 – http://www.sublimetext.com/ Super lightweight and versatile Text Editor. In my spare time I've been doodling around in Processing, but have found the IDE a bit lacking (no code completion, no snippets, no multiple carets). Sublime offers all that, and more. No matter if you use HTML, CSS/SASS, JavaScript, Rails, Java/Processing, Python or whatnot – Sublime has got you covered.

f.lux – http://stereopsis.com/flux/ Adapts the color of your monitor to the time of day. Less strain for your eyes. (Obviously, if you need color fidelity, this isn't for you)