Eclipse Trader looks pretty good but development doesn't appear active anymore- the last update was in '07. I have not tested this platform but I have used Eclipse extensively (as a Java IDE) and it is exceptionally good, especially for debugging. It seems like the Eclipse Trader team is centered in Italy, and they have some historical data sources for the Italian market. It would be interesting to test if algorithmic strategies are more profitable there than they are on the large, heavily traded US exchanges.

The ActiveQuant project looks much more vibrant. Here's their summary:

AQ is a framework or an API for automated trading, opportunity detection, financial engineering, research in finance, connecting to brokers, etc. - basically everything around trading, written in Java, using Spring. All is published under a usage friendly open source license.

They also have some good links to check out for java libraries and data vendors.

Thirdly, there is a fresh project named Marketcetera which seems to provide the framework for an API if that makes sense. At first I was enthusiastic but it doesn't seem very user-friendly (too early in development). This will be interesting to keep an eye on.

These were the only open-source systems I have come across so far that don't look half-baked and abandoned. I began looking because OpenQuant has not been as good as expected. The documentation is frustratingly lacking (and the dev. team [Dr. Anton et al.] regularly makes excuses for it in the support forum) and backtesting/optimisation are not flexible enough for my strategies.

I would like to hear you experiences with these open-source API's.