Flash Player is an extremely popular plugin for web browsers, that's obvious. What I find amazing about it isn't that its on nearly every computer that connects to the Internet, but that it gets updated so quickly. Recent stats show that about 80% of users update to the latest version of the Flash Player within 6 months of release. Its that reason that Flash is such a compelling platform to develop on, and why Flex is really taking off.

How does that compare to other technologies? Well, a recent survey has found that 40% of people aren't using the latest version of their web browser. More at Google, and the official study paper. That's a lot of people - more than 600 million of them. Some are running the latest main version, but without the security updates, and others are running older versions of the browser. (For eg. 92.2% of Firefox users were using Firefox 2, though Firefox 3 is available.) Updated by Mike: I misread the Google blog post referenced above.

How does this affect you? Well, with Flash Player, you can be pretty sure that 80% of people will be using the latest version within 6 months of release. As a developer, that means you can use all the newest features right away, without having to wait for users to update. That's very different than if you're developing apps in the browser.