<p>If you're a lover of <a href="http://db.tt/E21f4mT">Dropbox</a>, then I'm guessing you either already know about these three things, or you're going to wish you had! Each of these make for wonderful compliments to Dropbox.</p>
1. Send to Dropbox. Sign up and they'll give you a unique email address. Then, when you email an attachment to this address, it will get added to an "Attachments" folder in your Dropbox. I use this all the time when I am on my iPhone or iPad and want to send a file to my Mac. Most apps have email functionality, and I've saved my "Send to Dropbox" email into my address book, therefore making sending files to my Mac a cinch.
2. DropDAV. This is very useful for those iPad lovers who use Pages, Numbers, or Keynote. Those three iOS apps will connect to a web disk via WebDAV technology, which unfortunately is not supported by Dropbox. But fortunately, DropDAV exists. Sign up for an account, and they'll supply you with a web address to punch into iPad iWork apps, therefore allowing you to download and upload files to your Dropbox. [Update from Jun 2, 2011: Turns out DropDAV now charges $5 per month.]
3. Hazel. I love me some Hazel. Every Mac user needs it, but Dropbox users especially need to take the plunge since it can organize and cleanup your Dropbox files. Here's how it works. You install Hazel on your Mac, and it will monitor certain folders, waiting to implement actions which you set. For example, if you've already signed up for Send to Dropbox and are emailing in files to your Attachment folder in Dropbox, you probably want a way to automatically delete old files which you might forget about, right? Well, you could tell Hazel to automatically remove files which are three weeks old. Or, let's say you email in a PDF; Hazel could then move that file to a correlating "PDFs" folder. Hazel has plenty of other uses, but that'll suffice for the topic at hand.