StreamToMe is a great app that allows you to stream video to iOS devices. It also supports other platforms like Mac OS X. The way this app works is as follows: you install a server application on your Mac or PC that finds and catalogs your multimedia collection. This app is called ServeToMe. It then serves that content to any device running the StreamToMe app, which is the client app. ServeToMe transcodes all your content on the fly, so no conversion is necessary, and the file plays instantly on your device. Given that the content is streamed over the web or your home network, depending on how you set up this app, slowdowns and buffering may occur. Still, in my case, I only saw buffering once, and I wasn't running an ideal configuration.
The StreamToMe solution (server+client) can convert files of different formats into formats that play well on iPods, iPhones, iPads and Macs. If you have iTunes installed on your server Mac, you can also stream your iTunes music library, photos and even movies to any device that runs StreamToMe.
I have heard that the iOS version is great and it runs very well. The Mac version worked great for me.