closed as off topic by random♦ Jul 30 '10 at 17:16
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I've used Collabedit a few times and had good results, also Google Docs will do realtime shared editing as well.
one of the zillions of etherpad clones provide such a feature or you can grab the software and host it yourself, if you need it.
this question is a good candidate for webapps.stackexchange.com, btw.