Here's a doohickey I didn't even know Apple made.  At first glance it looks like a headphone adapter, but it's actually the iPhone TTY - a text terminal interface for relaying typed messages via an operator.  It costs just $19, but obviously you need to have access to the text-to-talk service if you're going to get any use out of it.

Note, even though it looks like a standard headphone socket, it's actually too small for a 3.5mm jack.

[via iLounge]