We have a legacy app that needs to be rewritten, and due to its size, we can only do it bit by bit over several years. The idea is to keep developing new features for the new app while still providing access to unimplemented features via the legacy app.
We're looking for recommendations for ETL tools that provide a GUI for the actual integration processes, for migration, etc. Ideally, the ETL can be done in real-time too, rather than in batches. Open source recommendations would be a plus.
Since your eventual target will be a java application it might behoove you to look into a java based solution.
You can find a list of ETL tools here.
As for open source, of those listed I've personally tested out a few over the last few years including Jitterbit, Kettle (Pentaho DI), Octopus, and ended up on Talend Open Studio for production.
Personally I found Talend's offering to be the quickest to pick up and supports a myriad of target platforms in either java or perl. Their community version offers everything you need to develop a solution, but their enterprise version offers a number of productivity tools you may find useful (job scheduling/shared repository/reusable joblets). I haven't made the jump to a paid solution yet, but after observing the community for a while I believe the product is very stable. On a side note developers seem very competent and are quick to implement suggested features.