Over the course of my career, I've tried a number of different Remote Desktop Tools - specifically for assisting remote users with technical problems. Unfortunately, in most of these cases you can not use the Microsoft Remote Desktop Client (which is otherwise EXCELLENT!), due to firewall restrictions and such.

There are load of these tools out there - such as GotoAssist, WebEx and so on. But I have always found them to be far too expensive. Many of them don't even publish pricing on their website which is even more disturbing!

Enter stage left: TechInLine.Com.
These are new guys on the Remote Desktop scene, and although their current offering has a few draw backs (moderate refresh rate), their pricing means they come out high up in the stakes as far as I'm concerned: $19.95 per month. That's it.
I've been making the most of a 15-day trial with them, and I'm fairly impressed; it's an excellent tool, it works well - traverses all the firewalls that I've got access to without a problem.
The only down side that I have encountered is the refresh rate could do with being a little faster, especially if you are working on a super fast Internet connection (why not sacrifice a little more ;)).

But, overall, very good - and certainly worth taking a look at!
Better than VNC, not quite as good as RDP in my opinion - and the price doesn't hurt at all.