Authentication solution

I've got an issue where I need to authenticate an external client/customer to multiple applications through our website. Ideally we want the client to only have to login once, but have access to all of the other applications as necessary.

Are there any security best practices available for this type of problem?

A single sign on solution has been discussed as a possible solution. Has anyone had any experience using single sign on with external clients on a publicly available website?

What problems (security or otherwise) did you encounter?

What other solutions are available?

Please poke holes in my ideas/problem, thanks!

Securing Apache Web Server with thawte Digital Certificate
In this guide we examine the importance of Apache-SSL and who needs an SSL certificate. We look at how SSL works, how it benefits your company and how your customers can tell if a site is secure. You will find out how to test, purchase, install and use a thawte Digital Certificate on your Apache web server. Throughout, best practices for set-up are highlighted to help you ensure efficient ongoing management of your encryption keys and digital certificates.;4175;25;1371;0;5;946;e13b6be442f727d1