http://www.callfire.com/
A whole heap of voice and sms APIs, in a similar vein to twilio / tropo, but seemingly a broader offering, and possibly a later entrant to the market.
https://ws.neospeech.com/
Interesting text to speech web service. Prices start at $75/mth plus 0.1c / minute then step up to $200/mth plus 0.01c/minute.
https://www.tropo.com/
Similar to Twilio, send / receive voice & sms via APIs. Doesn't appear to be as quickly evolving as twilio, but free for development use.
http://www.twilio.com/
Developer APIs for placing calls and sending SMS messages. Handles both incoming and outgoing, but only sends texts in the US and only has incoming US / Canadian numbers.