Imo there are a couple of things that you SHOULD DO after booting your mac, here are a couple of them that come into mind:
DO'S:

do keep iwork it will come in handy in almost every situation you will come up with in the future.

(if youre a student)in ~User/Applications/Utilities folder there is a program called Grapher, make an Alias(shortcut) of it and put it in your desktop. You'd be amazed at the versatility of that program.

Get VLC player, its probably the best video player program out there. I haven't come across any troubles with it in my 3 year mac exodus.

Get iStat pro widget for your dashboard.

(if coder)definately get Xcode from Apple developer tools. I honestly haven't seen a better coding software bundle out there. It has EVERYTHING. Even coding windows applications are easier on this!

(if hardcore gamer/overclocker)!!!USE AT OWN RISK!!!! If you adjust this too high or too low you will most definately damage your precious computer!!!!!!! Well unlike the warning its not that scary really. get smcfancontrol Its a pretty lightweight program which helps you adjust your fan speed at the cost of sound, the faster fan leads to a cooler mac, the cooler mac leads to a noisier mac, the noisier mac leads to Suffering...

Use Audacity for audio editing

Use Tomato Torrent for torrents

Use UnrarX(unrar program) for administrating the large amounts of parted files and errors.

Use Teamspeex for teamspeak chat client at Online Games.

Use Amsn for a Msn client(chat+it does have webcam support. but buggy. PRO: it can record your webcam conversation, CON: its buggy.)

Use Skype for video conferencing and chat(currently only official free program supporting video chat+ good quality)

Use Apimac Secret folder to hide your porn.

Use Flip4Mac to play .WMV's and Perian for a similar reason.

Use Cyberduck for FTP client

Use Gimp for free image editing. However i like Photoshop(eventhough its pricey, it gets the job done.)

Use iTunes, unlike a big group out there, i actually DO like iTunes. It does everything i want it to do and manages ALL my media(take that haters)
DON'TS:

Don't get ANY download manager software, AFAIC they all mess up your downloads.(do tell me if theres one you like. I'm DYING to know)

Don't use your mac with crumbly food or spillable liquid near it. Seriously Crumbs suck!

If you must use VMWare for windows emulation. But if your mac doesn't have a video card dont use windows at all.
This is all i can think of right now.