Are there any free online collaboration database schema tools?
You can use Vertabelo. Vertabelo is an online database designer working under Chrome and Safari developed by company I work for.

Vertabelo focuses on online collaboration in the visual database schema creation area. The most important feature of this tool is that it allow to share database models across the team and collaborate on them via web browser. It also allow to have public model link with possibility of embeding diagrams in your webpage/blog/wiki.

Others Vertabelo features include:


Support for PosgreSql, MySQL, Oracle, MS SQL Server, DB2, SQLite, HSQLDB
Dynamic/Visual search
Live database model validation
Model versioning
Forward engineering 
Reverse engineering 


Vertabelo is free to use for academic projects and have commercial versions for larger database projects.