Try LiveLOOK - Instant Screen Sharing, Co-Browsing, Web Conferencing - Windows, MAC, Linux. It is 100% web-based, which means it doesn't require software installations and works on any computer (Mac, PC, Linux). It is also very easy to add to yoru own website (as opposed to using from LiveLOOK - Instant Screen Sharing, Co-Browsing, Web Conferencing - Windows, MAC, Linux) - handy in case you want to do web conferencing directly from your website.