Adobe has finally taken Flash Player for Linux out of beta with today's release of version 9.0.31. You can download it from here. It is available in both tar.gz and rpm formats. Each is about 2.5 MB. If you use a Debian based distro keep an eye on your repositories. I expect we'll start to see deb files becoming available soon.
For years Flash has been one of the limiters that prevented many people from completely switching to Linux. Fully featured development tools weren't available and many distros don't even ship with the ability to view Flash due to it being non-free. And even when a Flash plug-in was added by the user there were still some sites that would not load because they required version 8 or higher. Adobe recently announced the Flash 9 for Linux beta to address the latter issue, but you might be surprised to know that a common application that may already be installed is capable of creating Flash presentations. I'm talking about OpenOffice.org Impress.