Receive & respond to SMS
One part of Taykt.com which I've not spoken much about is the web hooks facility built in. It means you can write applications which are prodded when a text message arrives, giving you a chance to decide how you want to reply. I can't think of an easier way get up and running with a SMS-based application: if you can write a web app, you can handle SMS, and you can do it without having to get out a credit card.
To fix the problem of keeping this so quiet, we've documented what we have at Assembla and started to pull together examples. There's a simple echo example (responding to a text message with what was texted in), but I have in my mind plans for other examples. My favourite would be to see The Internet Oracle implemented, although I also have a soft spot for Eliza and information services... the Taykt.com blog is collecting together ideas for how Taykt.com can be used.
We've also set up a RESTful API for managing accounts, trying to follow as much of the best practices as possible. But I think the web hooks part of the product is the most useful part to just be able to create something and have it running live.