Congratulations on getting a Mac. They're really nice machines, the OS is a joy to use, and the community is friendly
I'm not one of the Quicksilver fanatics. I actually like tangible manipulation of icons and things, so don't feel odd if you can't get into that program (or if you don't even quite get what it's for).
My favorite app is OmniOutliner. I use it for literally everything: rough drafts of writing, as a freeform database, for lists, todos, etc. I also second the recommendation for Camino. It's the best OS X web browser, based on Firefox, but with a Mac user interface. By default, some of the Firefox-type features are turned off (like "find as you type", keyword lookup in the URL bar, etc.), but they can all be turned on, either through options or downloadable preference panes. See www.pimpmycamino.com
for various add-ons if you find the defaults in Camino lacking. Oh, and you can get a single button for closing tabs, like old versions of Firefox, by customizing the toolbar.
If you need to keep track of your hours (for billing purposes, invoicing, etc.), there are a lot of good OS X apps. One that doesn't get mentioned very much and is somewhat hard to find is TimeLog. I think it's the best because it has fantastic iCal integration, a good, very usable UI, accessible through the menu bar, and very thoughtful reminder panes that pop up.
If you need a general alarm clock, timer, stopwatch-type program, Alarm Clock 2 is really good.
Some people have been mentioning concentration tools. One app that hasn't been mentioned yet, I don't think, is Desktopple Pro. A new version just came out. It lets you hide the desktop icons and optionally the title bar if you want to concentrate on a single window for whatever reason.