Mashups is a pretty broad term. I tend to think of three kinds of Mashup tools: front end, back end and integrated.
- Front end mashup tools: these tools help build web front ends using widgets/gadgets and little to no programming (iGoogle, PageFlakes)
- Back end mashup tools: these tools combine web-accessible data and services into useful web services (Kapow, Yahoo pipes)
- Integrated mashup tools: these tools can build end-to-end solutions linking web widgets to data and services
So you need to think about what kind of mashing you are trying to do:
1. Are you wanting to create a visual dashboard from existing widgets? Try a front-end mashup tool
2. Are you wanting to turn web-accessible stuff (like ebay auctions or linkedin contacts) into a web service API? Try a back-end mashup tool
3. Are you wanting to create an end-to-end web app like a dashboard or simple business portal? Try an integrated mashup tool
There aren't any full-blown open source mashup tools that I think really are practical in an enterprise environment right now. I attribute this to a few things:
1) Most mashup tools have a fairly sophisticated UI. In my general experience, many open source developers avoid this type of work
2) There aren't any real standards yet. Open source works great when the have a commercial product to model it after (heck, even Linux started this way).
3) There are already open tools and APIs for building simple mashups *outside* the enterprise... Most open source developers start with a desire to build something for themselves, not the corporate world.
4) A general lack of understanding of the potential of mashups is keeping them "low profile"
Those things said, I do expect this to change. Particularly if JackBe's EMML (Enterprise Mashup Markup Language) takes off. With a standard in place, I could see multiple projects kicking off to let you build/manipulate EMML in different ways. And maybe the vendors would all converge on it. JackBe is also trying to form an independent association among the vendors to drive this kind of consistency. In the end, I think it could be a great benefit for everyone.
As Chris points out, there are commercial products you can try for little or no cost. And there's also WS02, who say they have an open source mashup server. I don't know if they have any front-end tooling; I haven't used their product. I wanted to include them in my book, but their product was so new they didn't have any case studies as of a few months ago.