Welcome to the Mac! You’re going to get a lot of suggestions here, so I thought I’d add a list of non-application (mostly) names I think you need to know as a Mac user:
This forum. You’ve clearly already found your way here, but I encourage you to get the RSS feed. I’ve gotten better Mac advice here than I have anywhere else. The thing I find really valuable is that the advice has context – everyone here has a different style, and it doesn’t take long for you to find a person who works a lot like you (or like you’d like to), and come armed with good advice.
Screencasts Online. If you have the money for a subscription, this podcast is an excellent way to become a sophisticated Mac user pretty quickly. Host Don McAllister picks a concept a week, and gives you a thorough 20 minute video tutorial. This week’s: How to use your Mac Mini as a media center. Last week’s: Tweetie for the Mac. A few weeks ago: Numbers '09. This sounds like something a new Mac user like you might find very useful. (Even if you don’t get a subscription, keep track of the podcast – he offers the occasional free tutorial.)
Macbreak Weekly Picks. The weekly podcast is an acquired taste, but the show does feature some very smart Mac users picking applications and equipment they like. I’ve found their advice to be excellent. And expensive. This link takes you to a list of their weekly picks.
Curio. A really, truly great Mac app. Imagine a room filled with infinite, taggable, searchable, Evernote-compatible white boards. It’s a great mind-mapping tool, a great thought-organizing tool, and soon (if I have my way) a great index card solution. And it’s incredibly Mac-like: It’s beautiful, powerful, flexible, and it costs too much.
Services. You’ll find this menu option under the menu bearing the name of whatever application you’re in, and I encourage you to explore it. A lot of Mac apps use this feature, and it allows you to use certain features of those apps even when you’re working on something else. Devon Technologies (another name you should know) provides a few free servicesas well. I use their Word Service every day.
AppleScript/Automator You can get along really well without AppleScript and Automator – the application that lets you script certain repetitive tasks. Lots of people love their Macs without ever knowing these things exist. But getting good at scripting (or even functional at it) will take that love to a new level. Lurve. Lufth. Something like that.
Right click. The one Windows habit you should be sure to bring with you. AppleScript and Automator make the Mac’s contextual menus insanely useable.
Devon. The Omni Group. Mariner. Red Sweater. Rogue Amoeba. Flying Meat. A small list of very good developers. Names you can trust.