Today I got a note from the CTO of ScrumNinja, one of the ones I've played with a little bit, just checking in to see if we had questions for them. I sent a nice reply, explaining that we probably weren't going to be able to use them, pretty much entirely for reasons extrinsic to the product (company is a little too small and new, and not Windows-focused enough for us). But it did succeed in reminding me that they were one of the better-looking options in general, not least because they seem to be actively engaged with their user community, not just listening to but soliciting suggestions and ideas.
So now I'm thinking of trying them out for one of my open-source projects -- the Querki semantic-wiki projects, perhaps, or the Scala implementation of Wave. My quick kick of ScrumNinja's tires impressed me, so let's see how they do with something real. And points to them: little things like that feeler email are how you build your user base...