How do I migrate applications deployed on other application servers to GlassFish?

The Migration Tool automates the migration of J2EE/Java EE
applications from other application servers to GlassFish.
See the Migrate To GlassFish project page for more information about the tool and application migration, including a Migration Tool FAQ and a new Migration Guide.

Sekhar announced the new Migration Guide in this thread and also describes the guide and other Migrate To GlassFish happenings in this blog post.

As a first step in your migration process, you may also want to consider using the "verifier" (in the bin/ directory or every copy of GlassFish) to understand the level of portability of your application to be migrated. See this other blog from Sekhar.