The service is pretty simple to set up. You have to authorise it to access your Dropbox account, where it creates a shared folder. It then gives you a unique email address you can forward attachments to (I’d recommend adding this one to your contacts if you plan to use it much). You compose and send an email to the service and, depending on the size of the file, it’ll show up in your Dropbox folder in short order.
Habilis is a free web-based service.