They probably pay you more to write a couple of forum posts than the very small cost of any of these modules. Paying for a developer's hard work is just something one should get used to.

No problem, sorry to be snappy at you.

Even so, I'm afraid there is no free module for the purpose. The closest you will get is the user-sharing module from Innventua. It's not free as such, and people are expected to pay for it if it is used seriously, but the download is fully functional. The other two I mentioned are the only alternatives that will do user sharing. They're both more complex than Innventua's. DNNMaster's one can be used on trial.

Alternatively, if you run your portals all as child portals with aliases like "domain.com/child" then you can use the same accounts across them and normally stay signed in when you move between them. DNN user accounts of the same name must have the same password in different portals.. the accounts are not fully separated.

And also.. you may be able to reconsider exactly what you're doing and use a single portal to run it. If the sites are closely related then distinct site sections with different skins may be sufficent... for example, and Intranet for differetn business groups can run like that.

That's about the limit of what is possible

Rob