Four solutions that I know of-

1. Content Sharing - a third party module but it only works for a handful (something like 3 or 4) of the core modules.
2. Custom Build your own module from scratch
3. Use ListX to build the module - it will let you connect to most any datastore
4. Viva IFrame - third party IFrame module that will allow you to auto-login to another portal. You would build a page in "parent" portal (or pages) using Mitchel's free blank skin. Then use Viva Iframe in "child" portals to grab the module(s) from the parent. There are some security issues with this solution but if your content isn't super sensitive it shouldn't be a problem

Of course this all depends on what type of content you want to share. If it is simple suff like text/html then the content sharing module might be the way to go.

Greg