I’ve recently had a crack at using the Microsoft tool SharedView – It’s a competitor to The Citrix "GoTo" web apps and it’s free (nice).
It has some good points (It’s super easy to just share a single application, rather than an entire desktop, viewers can click on the presenters desktop to highlight areas to the presenter – good for support, it’s free) and some bad points (still not as easy as gotomeeting, with it’s 9 digit meeting ID and it’s a bit choppy… but hey, it’s free… it’s also in beta).
They seem to have "hidden" the app at the moment – it’s supposed to be a part of the "Live" suite of products, but there’s no mention of the software on Live (even in the beta section of live apps). I say, grab it while you can.
[UPDATE – 26/1/2010] – It’s now available publically – http://www.connect.microsoft.com/content/content.aspx?SiteID=94 – Thanks Jon!