I was skeptical of this web app, but Join.Me really works. Having said that, Join.Me is created by the folks at LogMeIn which is a fairly well known application that provides remote access to your computer - so perhaps it is not surprising that have developed this application.
The process is fairly simple... go to https://join.me/ and select SHARE. It will provide you with a 9 digit number. Other users then open their browsers and navigate to https://join.me/ and select JOIN. After entering the same 9 digit number they will be able to view your screen. I have only played with the free version which allows users to send messages, share files and remote control - that is pretty impressive.
Join.Me