If you're on a computer that doesn't have Dropbox installed, you can now upload files just by dragging them onto the Dropbox webapp.
You'll need to be using Chrome, Firefox, or Safari for this to work, but other than that it's pretty self-explanatory. No need to go through the tedious upload process, just drag your file into your browser window, drop it, and it'll appear in your Dropbox folder. Hit the link to read more.
Drag and Drop It Like It's Hot! [The Dropbox Blog]