I've been using StreamToMe to get streaming video/audio from my Mac to my iPhone and just recently I tried the iPad optimized version of same on my iPad 3G. StreamToMe seems to work very well on my iPad, very stable (not a single crash or significant problem) and the video quality over WiFi is certainly good but not great (more on that later).
I've also noticed a lot of recommendations for the AirVideo application in this thread so today I downloaded the free version (i.e. "AirVideoFree") and tried that on my iPad. AirVideo has a nice interface and the server seems to set itself up without any user interaction (that's nice) but in comparison to StreamToMe (and in my hands) AirVideo seems somewhat unstable. The video scrubbing feature seems to give the app all kinds of random fits (video will sometimes stop and it won't start again unless you exit playback and try playing the same video again).
However, AirVideo has one feature that allows it to deliver video that looks much better than StreamToMe -- AirVideo can stream content without conversion if the video is already in a format that is supported on the iPad. Furthermore, since there is no conversion this puts very little load on the computer that is sending the stream to the iPad (or at least that seems to be the case on my Mac Pro running Snow Leopard, however, on a G4 Mac mini running Leopard even when I've selected the no conversion option the Mac mini is maxed out on the CPU and the video takes a very long time to start playback). Unfortunately, even on the Mac Pro when I used the "no conversion" mode the AirVideo app still seemed somewhat unstable (when playing some HD files it even crashed).
In any case, AirVideo also has a "live" conversion option (converts video on the fly) which functions much like StreamToMe and this actually seemed to be the most stable mode within AirVideo. In fact, if AirVideo was as stable as StreamToMe it would probably be my choice between the two (particularly given the no conversion feature).
However, if StreamToMe offered a no conversion mode then it would almost certainly be my choice between the two. The StreamToMe web site says that it has a new version of the app awaiting approval from Apple that will offer additional options for streaming rates (probably higher quality). Furthermore, StreamToMe has some features that aren't in AirVideo. First, StreamToMe can output video from the iPhone/iPad dock connector so you can connect the iPad to a big screen TV (I used the iPad's 480p component out to view the video on an HDTV). Also, StreamToMe can operate in both landscape and portrait orientation (auto-switches when you rotate the iPod). AirVideo has neither of those features, it doesn't appear to support video out and it is a landscape only app.
To pick between the two, it's a little tough right now. But, at present I think I'd have to give the nod to StreamToMe (however, it's Mac only, but supposedly they are working on a version of the server to run under Windows). So, my recommendation for a video/audio streaming app on the iPad would be:
StreamToMe ($2.99, supports both iPhone and iPad, but presently the server component is Mac only)