There are basically two popular streaming media server such as TVersity and Air Video Server (AVS). However, I choose to use AVS as it support conversion ON-THE-FLY. In other words, you can stream and watch the video without needing to wait for the conversion to be completed. I believe AVS is the only media server so far that support such functionality. I didn't experience any lagging at all when running on Wireless G signal and AVS is running on Intel Core 2 Duo laptop.
Link for Air Video Server.
You'll also need to download AVS app for iPhone/iPod Touch from iTunes Store after installing AVS on your PC/MAC. There are both free and paid version of the app. The free version only allow a maximum of 4 videos to be shown on the list.
However if you wanted to try TVersity you can read the guide here.