Actually, there are two.
System i Navigator remains the standard – its free – and queries can be tuned in Visual Explain.
Or…you can go with the folks that invented their visual explain 4 or so years before IBM did, and have a set of tools that help you manage the queries, tune the db, optimize performance and more.
One of the newest modules, AutoDBA is worth a look to help manage unused indexes that have already been built (or shipped with the vendor app)
Happy holidays to all!!