There are a number of remote access apps that let you gain control of a PC or a Mac using your iPad. (Many of these apps also work with the iPhone, but I have found that accessing a computer on such a small device is best used only in the case of emergency. Navigating a high-resolution system from the iPad can still feel a bit cramped, but not so much that that you can't use it comfortably.) Remote access apps for the iPad range from the enterprise to the free, and I recently tried out three: LogMeIn Ignition, Splashtop Remote Desktop, and GoToMyPC. And check out Sharon Vaknin's video of TeamViewer, a great and free remote access app.
Of the three I tried, I liked Splashtop Remote Desktop the best, especially at its current discounted price of $4.99. It's easy to install and use, and it's the only app that streams both video and audio. The others, including TeamViewer, stream only video.
If the iPad is an integral part of your work life, however, I would suggest spending $30 on LogMeIn Ignition. It boasts two incredibly useful features: the ability to wake a sleeping or powered-down PC and the ability to copy or move files from a remote PC to the iPad. And before you dismiss it as too expensive, note that the $30 app does not necessarily saddle you with a monthly subscription. Other remote access apps, such as GoToMyPC, charge you each month or year.
Have a favorite remote access app not covered here? Let me know in the comments below and I'll check it out.