I've been tracking what browser plugins my eclipsecrossword.com site visitors have over the past few weeks. Overall, my numbers match up pretty well with the internet at large, with one exception—a lot fewer of my visitors have Silverlight than on the web at large. I'm thinking that this is mostly due to the fact that my site gets a disproportionate number of Mac users, since it's a product popular with teachers and schools still have a disproportionate number of Macs. (This is sort of sad since my product is Windows-only and therefore basically 100% of those Mac users are going to leave disappointed.) Silverlight is available for Mac OS, but it is significantly less popular than on Windows machines, at about half as many users.
Java used to be on just about every browser; now it's down to less than three-fourths, which is below my threshold for considering it a viable technology for reaching a broad user base. Silverlight, being much newer, is pretty far behind it, but in the same category—enough users have it that it's an interesting possibility for some kind of side project, but too many people would need to download it (and probably wouldn't) for me to depend on people having it. The difference between 50% and 75% isn't actually all that large to me: something's either a niche technology that I can't depend on at all under 35%, or it's broad enough that I could base a product on it at 90%, and anything between there is all sort of the same in the "interesting" category.
Flash is certainly dependable, but I'm not really that excited about building anything with it. I've had too many poor experiences with it (even more than Java) to make me care anymore. It might be the best choice from a "business" perspective, but as much as I sometimes pretend that they are, my side projects are not a business; they're a way for me to be productive while having fun and keeping my skills sort of relevant. That makes Silverlight actually the most attractive option of the three for building cool new internet stuff, despite the fact that it's the least common: it's in the same category of rarity as Java, and from what I can tell so far, a lot more interesting to code for.
Of course, I say all this like I'm actually considering building some cool new thing in my spare time, when in reality I don't have nearly enough spare time to start something new. I can only handle a few side projects at once, and more than one of my current side projects are add-ons for World of Warcraft. Those are a pretty good "deal" for my time—they're mostly fun to write, give me an interesting new perspective on development, and I get to actually use what I create, unlike my crossword builder or stuff I make for work.