Schema Mapping & ConversionWhile some folks design starting from RDF Schema and OWL, most applications in the Semantic Web will be derived from existing schemas and classes.
Relational DatabasesRelational Databases on the Semantic Web
Relational OWLRelational OWL is an OSS (GPL) Java application that converts relational database schemas to RDF/OWL. The conversion seems pretty one-to-one to me, so I'm not sure what the OWL is needed for. Also they need to connect to the database JDBC rather than just reading a SQL Schema.
SquirrelRDFSquirrelRDF is SPARQL server (using Jena) that supports querying most any datasource, and includes drivers for relational databases (via JDBC) and LDAP.
Open Source SQL SchemasThere are many interesting database schemas and datasets to mine out there.
The SDSS (Sloan Digital Sky Survey) has their schema browsable here: http://cas.sdss.org/dr5/en/help/browser/browser.asp.