Firefox plugins, Google Gears, Adobe AIR, Opera widgets, Yahoo Konfabulator, did I miss any other ones? Downloadable modules are a great way of delivering specialized software (e.g. Firebug), but they're horrible at integrating with existing networks.
With the web platform, we can easily send a link to a 3D chat app to a friend over instant messenger, hotlink videos on your blog, backup the bookmarks, etc.
Try sharing a Veoh TV video with a friend and you'll quickly find that learning how to use their UI and then convincing your friends to download and learn it as well is a major roadblock. Has anyone seen the demo map app made in AIR? I couldn't figure out how to for a good half an hour. Don't make me think.