There is Apple Remote free app from Apple.
-It allows you to control your iTunes on your mac. If you have your mac hooked up to your entertainment system wirelessly (through airport) or connected with a cable, you can control your music from your iphone. I love this app. I can DJ my whole house from my phone, it's great when guests are over! Since it's a remote control of your mac, it doesn't waste battery(you can turn quit the app when you want and the music keeps going) and you get full access to your entire music library.
-As a bonus, if you have Apple TV, this same app will allow you to control your Apple TV. You can actually use the iphone keyboard to enter text into your Apple TV!
As far as wirelessly syncing contacts, calendar, bookmarks, etc between iPhone and Mac, you'll need a MobileMe account. It's pretty neat when you get it set up. I can edit a contact or calendar on my phone or mac and it'll update on the other automatically. This works from anywhere there is a 2G/3G signal on your phone. Pretty cool.
Note that these aren't specific to Macs only though, I think they work with PCs as well. I'm sure they work better on macs though
Kan-O-Z