Every time I start planning for a web site or a web application, I end up with some feature I really like from some opensource software, and I wonder "can I just use it?". The answer, 99%, is no, plain and simple.
That's because most open source developers just don't think about integration. You are coding a (possibly) small software, meant to do just one thing: KITS, right?
Wrong. We're not in an unix mainframe environment anymore. We NEED interoperability and accessibility to all the services. We need integration. And we need embeddable software, not just code, be it an opensource project or not.
Web services (even simple REST enabled services) are a huge step forward, not 100% ok maybe, but yet...
Many programming frameworks (like cakephp) are already providing a web services infrastructure you can leverage to implement a webservice with ease. Do it, next time you start coding.
Meanwhile, some important and business oriented softwares are indeed "going open" - via ws API or inclusion point: KnowledgeTree and eGroupWare, just to name a couple, are both exposing an web service endpoint.
Others are not doing the same - phpBB, one of the best known forums, is a nightmare to integrate.
So? Next time you choose, or write, or buy a software be sure to check how OPEN it is. It doesn't need to be OpenSource, but it HAS to allow for easy integration. It's mandatory nowaday, and it's not about the stupid SOA hype. It's about IT 2.0.