Revelytix Spyder is a free product for accessing various data sources, including databases, with SPARQL. It supports W3C's R2RML mapping language. Commercial support is available. Revelytix publishes a comparison of Spyder and D2RQ.
Virtuoso RDF Views has pretty smart rewriting algorithms and also supports Named Graphs and W3C's R2RML mapping language (details). Commercial support is available.
Sparqlify is an experimental RDB-RDF mapper. It was developed to allow SPARQL access to the OpenStreetMap database. It supports PostgreSQL databases and uses a concise, proprietary, SPARQL-based mapping language. It rewrites any supported SPARQL query into a single SQL query.
ODEMapster is a database-to-RDF mapper that uses the R2O mapping language.
Triplify, a small plugin for Web applications, reveals the semantic structures encoded in relational databases by making database content available as RDF, JSON or Linked Data.
SquirrelRDF. In addition to relational databases, SquirrelRDF also supports access to LDAP directories.
METAmorphoses publishes RDF generated from relational databases on the Web.
DartQuery. DartQuery is a component of the DartGrid application framework which rewrites SPARQL queries as SQL against legacy relational databases.
SPASQL is an open-source module compiled into the MySQL server to give MySQL native support for RDF.
The W3C RDB2RDF Incubator Group was chartered with the mission to examine and classify existing approaches to mapping relational data into RDF, as well as to mapping OWL classes to relational data. See their State of the Art Survey and Final Report.
Berlin SPARQL Benchmark comparing the performance of D2R Server with other database-to-RDF mapping tools and RDF-stores.