XModeler (originally called XMF-Mosaic) is a meta-modelling platform that supports a range of modelling activities including:
XModeler is available under the Eclipse Public License and is available to download here (id: dev, password: dev123).
Once downloaded, there is extensive documentation available by clicing on the Getting Help link as shown below:

The help system should start in your browser as shown below:

XModeler runs an embedded instance of the XMF language engine (the console window in blue above).
XModeler can also be used as an IDE for developing XMF applications so check out the XMF documentation too.