Air Display 1.6
It’s not cheap by App Store standards, but certainly cheaper than another monitor. Once you purchase Air Display for the iPad, the Windows or Mac counterpart is a free download. The downside is that this second monitor must be on the same Wi-Fi network as your PC or Mac. While HD video is jerky, ordinary video refresh is pretty quick -- very acceptable for most nonvideo applications.
Developer: Avatron Software
Compatibility: iPhone and iPad; iOS 4.3 or later