Quite possibly my longest post on a forum.
Wow, I was about to start a thread like this when I saw this one. I too just ordered my first Mac (see sig); it's not here yet...

Anyway, I have read up quite a bit on Macs.
Golden Rule: It's "Mac" not "MAC". For some reason people see "PC" and assume "Mac" is an abbreviation too... or something.
Here's a list of Mac apps I have compiled for me to install on my Mac in no particular order (all free):
Firefox (duh)
Transmission Simple BitTorrent client. Not that I'm recommending you pirate things; there are legitimate uses.
Hulu Desktop Hulu as you know and love, but as a standalone app.
Quicksilver This is supposed to be the cat's meow. I have used Gnome Do (Quicksilver clone) on Linux, didn't like it too much, but maybe It'll grow on me? That's what I've heard.
Adium The de facto Mac Instant Messaging App.
Colloquy Alas, Adium doesn't
do IRC... yet. Use this app instead for that.
Unarchiver Need to open a .7z? What about a .tar.gz? The Unarchiver has you covered!
Butler An neat little thing you might find some use of. See the website.
Burn If you simply need to throw some files on a DVD, here ya go!
Growl Another Mac favorite, Growl allows you to get cool little notifications about whatever an application wants to notify you about, such as dowloads completed, IM/mail received, current song playing, etc. Sort of like notification ballons in Windows, but much more nice looking.
Displaperture Not really useful, but cool nonetheless. It allows you to round off the menubar (a la pre-Leopard) or any of the four screen corners you like. It can complement the black trim around your screen (see
here.)
AppCleaner When you drag an app to the Trash, there may still be some "straggler files"; .plists, cache files, etc. This app attempts to completely remove apps when you drag them on to the window. Can also be configured to activate when you move an app to the trash. One of the few free ones.
Flip4Mac Windows Media Components for QuickTime QuickTime plugin that allows it to play WMVs, mainly. Microsoft endorsed.
OpenOffice.org/
NeoOffice Whichever you prefer. OpenOffice.org is a free office suite that runs on all major OS's (Win/Mac/Lin) and is mostly compatible with Microsoft Office. NeoOffice is a version of OpenOffice.org made to be more like a Mac app.
This one isn't free:
CrossOver. Sort of like Parallels or VMware Fusion; allows you to run Windows apps without booting into Windows. The difference is that CrossOver is a implementation of the Windows libraries, think of CrossOver as a "translation layer" between Windows apps and OSX. CrossOver is a proprietary implementation of the open-source Wine project, but sales from CrossOver fund Wine. Keep in mind Wine has much less spit and polish and no Cocoa interface.
Here's some Firefox addons, not really Mac specific, that you may wish to try:
Greasemonkey The addon.
Stylish Like Greasemonkey, but centered around changing the look of websites and/or Firefox.
SmoothWheel The best smooth scroll addon I have tried.
Surf Canyon Helps improve Google search results, among other things.
Organize Status Bar Let's you rearange the status (bottom) bar.
Tabs Menu Makes a menu in the menubar listing all the tabs open; thought this might be cool on a Mac.
Stop-or-Reload Button Hides the Stop button when page is not loading, hides refresh when it is, a la Safari.
SearchBox Sync Sync what your current search is in a search engine with the text in the upper-right search box.
Lazarus Saves long forms so if Firefox crashes your tedious MacRumors post doesn't go up in smoke.
StumbleUpon Select your interests, press "Stumble!" and wham! a page that (should) be interesting to you. Press "thumbs-up" for more like it and vice-versa.
Locationbar2 Makes the address bar Google Chrome like, highlights the website and grays out the directory and subdomain. Also linkifies it when you hold ctrl, shift, cmd (I think), or alt.
This one is Mac-specific:
Keychain Services Integration. It's supposed to integrate the OSX keychain into Firefox. It's still experimental, so beware. But if anybody is willing to try it, perhaps on a seperate FF profile, please let me know how it works.
Whooo, long post. Maybe I'll clean this up and make a thread out of it.