What you can build with Eris stack?
You can start your own blockchain (no point of course to keep it all for yourself - for the members of a particular system) The chain can be verified against Bitcoin, Ethereum or any other big public blockchain.
For example, you can have a blockchain for the players in the multiplayer game, a cooperative, an association of legal entities or projects in which people have to manage assets TBA
These chains can be used to run Dapps and build DAOs.