If you work with WordPress 2.6 or before and have recently installed Flash Player 10, then you probably realized that the upload dialog box has stopped working. This is due to a security update the Adobe made in the latest version of the Flash Player.
I have seen a lot of people complaining about this (and Adobe has responded really well and explained their solid reasoning), but the fix is actually really simple and takes only two minutes to install. Just download the ZIP file mentioned in this blog comment on WordPress.org, copy the included files over your current installation, and you'll be all set. The fix includes a new version of SWFUpload, which is used on the WordPress upload page in the admin panel.