TSclientX is a Windows Terminal Services client for Mac OS X. It appears to be a regular Mac OS application but is in fact an assembly of free software working in concert to provide similar (better?) functionality to the official Remote Desktop Connection from Microsoft.
Additionally, there is a nice tidy VNC Viewer built right in as well.

What makes TSclientX interesting compared to other Mac RDP Clients?

- SeamlessRDP is brought to you by Cendio's SeamlessRDP component.
It was originally introduced in rdesktop 1.5 and just simply rocks.
HINT: Download Cendio's SeamlessRDP component from
http://www.cendio.com/seamlessrdp and extract to C:\SeamlessRDP
Check the "Fullscreen SeamlessRDP" box in the Display tab and
in the Programs tab under "Program path and filename" type in:
"C:\SeamlessRDP\seamlessrdpshell.exe taskmgr.exe"
(Without the quotes, you'll get the idea quickly enough)

- Multiple instances of TSclientX can be easily started.

- Stability and speed on Intel Macs (and PowerPC Macs for that matter)
has been found by many to be superior compared to the official
Mac RDP client.