Since the Mail app has become a bit temperamental in Mavericks I have switched to Airmail which seems to be working so much better for me
I got Airmail when it launched, and it's fantastic! I can't see myself Not using it anymore.
Another one I use all the time is 1Password, I don't know how I'll manage without it anymore. With all those large websites and services getting hacked, it just makes so much more sense to use it, and at least a 14 character randomly generated password.
I've also been using the Parallels Desktop 9 trial, and I'll most likely buy it. It's Coherence mode is fantastic at blending Windows applications into OS X. Making them look as they're apart of OS X, and it can easily share your common folders with each other as if you're still just using on OS.