An unincorporated association is the simplest form of a non profit organization. DIY University is an unincorporated association. My research has shown that unincorporated associations can grow into international organizations and that they can be turned into for profit ventures too.
Here's what nolo.com says about unincorporated associations.
What happens, legally speaking, when a group of people get together and decide to perform some task without filing any legal paperwork or establishing any formal legal structure? Whether they know it or not, they have formed an unincorporated association. Unincorporated association means an unincorporated group of two or more persons joined by mutual consent for a common lawful purpose, whether organized for profit or or not.
Linder Myers solicitors on unincorporated associations.
There are many situations where a group of individuals come together to further a common aim or interest such as a sporting or other social activity, educational or community improvement.
( I purposely underlined part of the sentence above. DIY-U is an educational project which aims to have a positive impact on education as a whole. )
Brookes Pierce and unincorporated associations.
The may include collectives and online communities. There's no end to them: groups with common purposes that are not incorporated.
(My analysis of the sentence above: DIY-U is an online community. We may form physical world 'brick and mortar' groups too, in the future. Unincorporated associations may influence off shoot unincorporated associations or they may just transform over the course of time.)
Here's a youtube video on unincorporated associations.