iSeries DB2 - SQL GUI Tool
From my own research SQuirreL SQL is the best open source GUI client for the iSeries. It tried it out today it it worked nicely, code completion, syntax highlighting etc.
I highly recommend DbVisualizer with the IBM Toolbox/JTOpen database driver. You can also use it to connect to almost any other database as well. It's cross platform and runs on Linux, Mac OS X, and Windows.
There is a Free edition edition for commercial and non-commercial use.
IBM provides IBM Data Studio at no charge. http://www-01.ibm.com/software/data/optim/data-studio/