1. Quicksilver - There are already explanations on this and I don't like Spotlight that much.
2. MacPorts - If you are a developer or you're just an open-source lover, this is a must have app for you. It has 5811 open-source programs ready to install, easy and... open-source! ( which is good... )
3. TexMate - My every day editor, I don't use another one besides this one.
4. Adium - For chatting.
5. pgAdmin - For PostgreSQL administration. I don't use MySQL that much.
6. Growl - Another must have for system notifications, I don't realy know why Apple hasn't put one in OS X yet.
7. OmniGraffle - For Visio projects and such.
8. Things - For task managing.
9. Tweetie - The best Twitter client I've ever found.
10. Transmission - For P2P file transfers. I don't like uTorrent very much.
11. AppZapper - To uninstall programs. When you delete an app some configuration files are left behind, with this program all the files related are also deleted.
12. Evernote - Well... to take notes! Even better if you have an iPhone.
13. VMWare Fusion - For virtualization
14. Scribbles - For those moments when you get inspired and just want to make some draws :)
15. Adobe - Photoshop and Dreamweaver between others are a must.
16. Teeworlds - Best game ever and... Open-Source!!!!
17. Toast Titanium - For recording CD's and DVD's.
18. ScreenFlow - To record your screen and make nice video editing
These are not by "like" order, they're by the order I remembered them.
I wouldn't recomend installing iTerm because Terminal.app does the job nicely and it has tabs as well.
I'll had more if I remember some.
[0] https://gist.github.com/vmsp/38b9a891c76a2fc5c3b5
[1] https://gist.github.com/vmsp/5cbfa1b4ccd9f78f56c7