Hello.
I use rdesktop,
http://sourceforge.net/projects/rdesktop/
Once it is installed you simply type:
rdesktop -u <username> <IP or name>
e.g.
rdesktop -u daniel boyfarrell.com
If you don't like the command line there is a GUI called tsclient that you can also install.
The best way to get unix/linux apps for you mac is my installing darwin ports,
http://darwinports.opendarwin.org/
Once darwinports is installed you can get a huge amount of free software. You would get the above by typing in the terminal,
sudo port install rdesktop
sudo port install tsclient
Then of course you could just use Mircosoft remote desktop client for mac,
http://www.microsoft.com/mac/otherproducts/
Dan.