Remoter™ VNC: Remote Desktop is a Chock-Full-of-Features, and also the most aggressively priced VNC (Virtual Network Computing) client for iOS devices. It’s a Universal Binary too, meaning you buy it once, and you can use it on your iPhone, your iPad, and your iPod Touch too!
VNC is the Industry-Standard for remote desktop, and is already integrated into Mac OS X. There’s VNC servers for all versions of Windows too, and also for Linux.
Remoter is very easy to use and intuitive!, but in case you need help, it also includes an in-app manual with instructions on how to use and set it up. For more help, there are forums available.