On a high-level, there is the asynchronous, unmoderated, remote
system, available for a wide range of prices. Then there is a more do-
it-yourself synchronous, moderated, remote approach that's essentially
a screen-sharing & ideally recording solution, also available for a
wide range of prices. Plus Morae is on the list, as a step towards a
more complete overview of usability testing systems; Morae also plays
well with UserVue.
I'd be glad for people to expand on this material, which is a publicly
shared Google doc. I'll update it as I get more information from a
couple vendors, too. Enjoy!