We are living in the golden age of free web API's. Ever since Google started the trend by giving web developers free access to maps, many formerly expensive services are now available free to try on a non-commercial basis.

An API I was excited to discover was one from http://voxeo.com They offer free (of course) hosted Interactive Voice Response (IVR) system. An IVR system is what runs all those "voice jail" systems that everyone loves to hate: "Press one for... Press two for..."

I mashed up the local robo-radio stations website:
http://www.bostonradio929.com/just_played.aspx
Scraping their page and serving up the last song played in VoiceXML. Give this number a call to hear the result:
415-830-5441
The page that generates the VoiceXML is here:
http://gadgeteer.org/vwbos.php
The phone number of the person calling is available to the VoiceXML code and many more things are possible like branching on key presses or voice commands

Voxeo is offering is a free try-before-you-buy offer. Once you are ready to realease something into production prices start at $0.11 per minute with a minimum of $500/month

Another API that makes something that was once difficult and expensive free is http://www.textmarks.com Register a unique tag with them and any time someone texts that tag to 41411 your server gets called with the phone number passed as a paramter. Try texting "wbos" to 41411 and again I scape their page and send back the last 3 songs and artists. TextMarks allows you to use their service on a commercial basis as their business model is to embed ads into the response messages. Hopefully they also cut a deal with the carriers to get paid for the SMS traffic they generate. If you run a service that generates enough text messages that people actually want to get (not ads) the carriers will actually cut you in for some portion of the .15 per message that they collect.