Air Video is one of the best apps to stream video content from your computer to your iOS device (iPhone/iPad). It plays almost all the common video formats with a very simple interface.
The app is available in the Apple's App Store as Air Video Free. A Video Streaming server software needs to be installed in the computer.
The app has a built in transcoding which it does on the fly. The video will be converted to a playable format automatically.
More details can be found at the Developer's website. The video streaming server software is also available from the same website.