I too have been searching for something like this. The answer, like the previous poster states, is that there are many such items out there already. The problem is tailoring something that is already out there to suit your needs.
The best solution for an extranet is your own. Many of the solutions revolve around 'groupware' solutions that basically provide a calendar, discussion forum, file repositories, etc. This may be what you are looking for. If so, simply do a search on freshmeat.net for 'groupware' and you will find many of them. Most require php4 and mysql (which is not a problem) some even allow you to choose your database between postgresql and mysql, and others might simply require ODBC support.
The problem you will run into, if you are not that familiar with PHP and database type programming, is setting it up to meet the specific requirements of your clients. If you are familiar with programming such applications, then altering such a program might not be too dificult.
I found that most of these 'groupware' products offered many great items, but none that had everything I needed. I therefore downloaded the ones that had the content I wanted, and am currently building my own out of the pieces. (this drastically reduces the development time)
Anyway, didn't mean to drift, but I hope this provides you with some more insight in how to cobble together an extranet.
Oh, as always, mind your security when doing this. Most clients want SSL connections for anything valuable! Data theft is rampant if you don't pay attention.
Sincerely,
Charlie Crawford
