Depends what you're wireframing really. One thing I've been using lately for database modeling is MySQL Workbench 5.0 OSS which is an amazing free tool. Just google it. It is a must download in my opinion. Check out the EER Diagrams feature. You can model your database by hand or import an existing SQL database file which will render the diagrams for you, allowing you to edit, create table relationships at will etc. You should be able to output your diagrams/wireframes from here...
But I have a feeling that you might not be after database wireframe software?