A few things I use most on mine:
- Microsoft Office 2011 (other office packages are available including Apples own stuff but none of them are in the same league as MS Office. Microsoft did a nice job on the 2011 Mac version).
- Parallels 7, this is great if you want to run any Windows apps (I launch the Bootcamp partition as a VM) or have a play with Linux or various other OS's.
- Steam, yup, you can actually play games on it!
- Sparrow, great mail app
- Skitch, great screen grabber
- Pixelmator, nice little graphics app
- MAMP, sets up the Apache, MySql, PHP environment for you
- Handbrake, get your DVD's into iTunes
- iBooks Author, get your novel on the app store
- TextWrangler, nice plain text editor