I managed to sign up with Yugma and within a few short minutes I was already set up and able to share my desktop with a colleague. The free Yugma service allows me to invite up to 10 attendees and thats more than enough for me and I think its a pretty decent web collaboration tool.
I had a quick look at two other tools that give free desktop sharing. The first one is Yakkle which competes very well with instant messaging tools and remote meeting and web conferencing tools. It would be interesting to hear from other people who use the tool more often.
The other free tool that I came across is Vyew. No installation is required for this one and therefore you are able to use it from any computer with a browser. I didn't get around to test it but I hope to find some time to give it a go.
There are probably a few other tools and if I come across them I will add them to the list.