My employer produces a simple, proof-of-concept HTML5-based SQL client which can be used against any ODBC data source on the web-browser host machine, through the HTML5 WebDB-to-ODBC Bridge we also produce. These components are free, for Mac, Windows, and more.
Applicable to many of the other answers here -- the Type 1 JDBC-to-ODBC Bridge that most are referring to is the one Sun built in to and bundled with the JVM. JVM/JRE/JDK documentation has always advised against using this built-in except in experimental scenarios, or when no other option exists, because this component was built as a proof-of-concept, and was never intended for production use.
My employer makes an enterprise-grade JDBC-to-ODBC Bridge, available as either a Single-Tier (installs entirely on the client application host) or a Multi-Tier (splits components over the client application host and the ODBC data source host, enabling JDBC client applications in any JVM to use ODBC data sources on Mac, Windows, Linux, etc.). This solution isn't free.
All of the above can be used with the ODBC Drivers for Sybase & Microsoft SQL Server (or other databases) we also produce ...