I am one of those who like instant desktop sharing for free without installing any software or configuring user account at the target PC.
This may sound bit greedy but I found some good solutions to it.

1. Adobe ConnectNow.I fond it most real time and fast and smooth if there is a broadband connection.

2.Web site Vyew (think view) lets you collaborate in real-time with other users in a web based meeting room.You can either start an anonymous meeting (requires no registration) and invite up to 2 users, or, if you go through the free registration, you can invite up to 20 participants and get a few other members-only benefits. Vyew offers photo sharing, whiteboarding, file sharing, screen captures, and can run from your browser without the need to download anything (sort of). Vyew requires Flash and Java.

3.Zoho Meeting is one of large number of free collaboration tools which does not need any extra installation.



However beside above these free tools I find Oracle Web conference most real time and efficient tool for desktop sharing.

Among paid services  GoToMyPC is the right solution if you are into small business and want the best productivity using screen sharing.