There is a kind of alternative that I tried to use called StreamMyGame, which is a cheaper alternative. Instead of streaming from a third party computer over the internet, it lets you stream over a network instead (Though internet streaming is possible).
However... its fairly naff. You need to pay a subscription if you want to get decent resolutions, which is fair enough, but the software doesn't seem too good. The free trial doesn't show up many of the problems since you're playing in such a small window, but even on my 130mbps wireless network at 1280x760 WoW is unplayable, its like its not sending the information fast enough or something.
http://img831.imageshack.us/img831/5199/stream.jpg
If it worked properly, it would be awesome, as I could play games on my laptop and htpc easily, just streaming them from my gaming PC upstairs, and unlike onlive I'd be able to run whatever mods I'd like, and as you're dealing with a LAN, there would be no real latancy issues. Apparnetly some of these issues are 64-bit issues, thoguh either way their website is pretty awful, and dev feedback is pretty much nil, they are silent, and I don't think its been updated in years.
So... this is probably the best alternative, but I wouldn't advise paying for it as the devs don't seem to care about it. If someone could create something like SMG, and just make it work, I'd happily pay for it.
Also, I should note that SMG does let you stream games over the internet like OnLive, but I imagine this would be awful compared with OnLive's service because nobody's upstream bandwidth would be able to stream the video at a decent resolution.