The one I like the best is:
iTap RDP
Second
iTeleport Jadu RDP
Third
Jumpdesktop
Good cheaper alternative
RDM+
For some of these you will need to configure Port Forwarding.
For RDM+ you will need to download a free client that runs and enables you to connect from 3G
For Jadu RDP there is also a client.
NOTE: Jadu RDP does not have an optimized app for the iPad. So if you plan on Jadu check their website. They have other products that use VNC and I think may be universal.
I have not used this, however I have heard good things about the priciest app, LogMeIn at 29.99. I hear the setup is easy.