What's a good low-res to HD converter for Macs?

Hi Guys,

I've done a lot of hunting on the internet, and most of the applications I've found to do this job...well, don't do that job. I'm just looking for an app that lets you convert low-resolution (480p, etc.) videos to HD (720p, 1080p, etc.) quality. If anyone has had the same issue, and has found the perfect app, I'd love to hear about them. Thanks!