Dockdrop isn't necessarily something new to Mac OS X, but it's a simple and a cool application none-the-less. Dockdrop allows you to drop files on top of it and perform an action. You can use the following protocols to send files: FTP, Flickr, WebDAV, or SCP. Dockdrop reminds me of the "droplets" that you can create in Panic's Transmit FTP application (and is pretty much the same). I tested the FTP and Flickr upload functions and they both worked seamlessly.
Dockdrop works with both Mac OS X "Tiger" (10.4) and "Leopard" (10.5). You can download Dockdrop for free from the developer's website.