Top Producer (an expensive option) is also available as a web-based program... which should (I've never tried it) work online with Mac.
Many so called "online" programs are still not Mac compatible. They are set up to work with IE. Using a Mac for Real Estate is frankly the best way to go. Congrats on going that direction. The best contact management system designed specifically for the Mac is R.E.S.T. It is designed using Filemaker so it is a relationship d.b. and is beautifully laid out and simple on the start up, which is one of the reason we use Macs.
R.eal E.state. S.uccess T.ools has fully integrated with all your existing programs including mail. I think you will love it. Email Karen Hardy to find out more email@example.com.
In all honesty, I do not look to a Mac for business related tasks but anything that is web based should work no matter what operating system you use. On another note, I do hear that the address book that is built into the Mac is fairly adequate.