There are several alternatives, many of them free, depending on the functions/features you want/need... a lot of software you'd never expect also allows sharing either video or a video feed, like VLC. Check out Videohelp.com. Performance also varies with your hardware, if you're planning on using tuners etc. live from within the app.
GB-PVR is interesting, but a bit more hassle to set up. Cyberlink had a decent setup, but they're going to start charging for it. Beyond TV is more popular I think than Sage if you're going to pay. The easiest may be a downloadable, bootable, Linux disk image (forgot the name - sorry), where you just boot into an HTPC eviron.