The Oracle Migration Workbench (OMWB) is a free tool that simplifies the process of migrating third party databases like Sybase, Informix, and DB2 databases to the Oracle platform. The Oracle Migration Workbench migrates the entire database schema, including triggers and stored procedures, in an integrated, environment. It can also perform the data move either online using JDBC or offline using scripts.

The OMWB is currently in transition from a standalone tool (OMWB 10.1.0.4) to an extension of SQL Developer (SQL Dev 1.5).

Informix & DB2(UDB and iseries) are still supported by OMWB 10.1.0.4.
Microsoft SQL Server, Sybase, MySQL and Microsoft Access are now supported by SQL Developer 1.5

More details and downloads can be found here
http://www.oracle.com/technology/tech/migration/index.html