when i do a new install of my mac operating system or get a new machine, here are the apps that i MUST install in order for me to use my mac properly:

1. quicksilver, the best app launcher and so much more
2. dropbox, the best online storage to back up, share, and sync files. awesome.
3. xmarks, syncs bookmarks between all your browsers and across machines. backup for your bookmarks, basically.
4. 1Password, a single repository to keep all your passwords. 1password, along with dropbox makes it easy to have strong passwords to all your favorite websites, but only have to remember one password to access it. using dropbox, you can have the same 1password settings across different machines. also a cool iphone version.
5. evernote, a do-it-all "scrapbook" for you to store almost any kind of file. throw important website, documents, images, pdfs, etc. on your evernote account, and you can access it anywhere. they have this cool text image search function that finds texts WITHIN images. good iphone app, too.
6. textmate, a really good text editor. text edit that comes with the mac is good, but textmate takes it to another level. i do a lot of text editing of xml, html, etc. documents, and textmate is awesome.
7. vlc, a video player for the mac (and windows/linux) that plays almost any kind of file. quicktime on the mac is pretty lame when it comes to playing different file types. vlc is the solution.
8. mouse locator, for today's larger screens, it's easy to lose the mouse cursor. mouse locator puts a colored ring around the pointer for a few seconds to help locate it...sounds silly, but very very useful.
9. perian, which provides quicktime with more functionalities to play different media types. still doesn't play as many filetypes as vlc, but a must-install for any mac.
10. super duper, to clone your hard drive to an external drive. you need to back up your hard drive, and more than just back up certain files, why not clone the entire drive so that you can have a bootable backup drive in case you need to boot from it.

the great thing about almost all the apps above is that they are free. the only ones that cost money are 1Password and textmate. the free alternative to textmate is textwrangler from the guys who make the excellent BBedit. a lot of mac developers use BBedit as well...between textmate and BBedit, i bet that accounts for 90% of the mac developer community.

a couple of good apps to use for you video people are handbrake and transmission. both are free. handbrake allows you to rip DVDs so that you can save them and watch them on your computer or ipod/iphone. it also converts .avi and .divx (and other formats) into quicktime-compatible formats so that you can watch on your mac/ipod/iphone. very powerful and very useful. transmission is a bittorrent client that was developed by the same guy who wrote handbrake. it's now an open-source project, but it's very good.

that's about it. install the above apps and your mac is powered up and ready to go!