I want to add a dashboard functionality to an existing java web application, this should be a portlet container, so it can host other mini applications or open social gadgets.
What is the best solution for this? Integrating an existing full blown portlet container?
Look at Liferay as a portal container:
http://www.liferay.com/products/liferay-portal
The open source version is free, and there's an enterprise edition (if you feel the need).
Apache Pluto is an embeddable portlet server if you're looking for a simple engine to use existing portlets - it even has a JSP taglib you can use to embed in views.
You mentioned OpenSocial though (which I'd recommend over JSR-168, unless you have a particular portlet in mind) for which you could look at Apache Shindig, though that's still somewhat of a moving target.
Infopark has a JSR-168 Portlet Container delivering portlets anywhere in normal content. They do content management, so the portlets are a way to create personal dynamic parts in static content. It works as a webapp in any servlet container like tomcat 5.5. Sorry, but it is commercial software.