It's not as convenient as someone else providing it for free, but it you purchase VMWare Fusion for Mac, you can download and use one of the multi-browser virtual applicances from VMWare:
http://www.vmware.com/appliances/directory/cat/0?k=browsers&c=0
This will give you a one or more virtual machine environments which you can fire up any time to test your site live.
If you prefer, you can build up your own virtual PC using Fusion and install all the browsers you need into it. Or just access the doesn't-work-on-a-Mac sites given by other answers.
If you don't want the performance hit (uses a bunch of memory and CPU) of running a virtual machine, you can always buy a cheap PC and install all the browsers you care about on it, keep it connected to the network somewhere in your office and use MS Remote Desktop client for Mac, VNC or www.logmein.com from your Mac to connect to it as and when you need if physical space is at a premium.