Well, our favorite telco have a publically accessable DSL availability checker at http://www.dslchecker.bt.com/adsl/
Until now we have provided a checker on our we site which uses a specific API to them to get the data and present it to customers on the web page. We use this as part of the ordering process to validate the line and offer the right services too. We also used to use it to proactively check for FTTP availablility and 21CN upgrades on lines (a low level of periodic checks over night to be nice).
Saldy, in their infinite wisdom, they started charging 60p a lookup if we exceed a pretty low level of usage! So we changed to caching replies (causing problems with out of date data) and not checking for upgrade opportunities- losing them revenue in the long run.
Seems this was not enough reduction, and so they are billing us around £2,000 a month. Madness!
So the availability checker on our web site now links to their publically available site, and even puts the results in an iframe, so looks almost exactly the same as before.
We don't see the results as it is the end users browser that actually does the lookup, not us, so if someone does then order a line we do an extra lookup, increasing the load on their checker, but at least we don't get to pay 60p every time someone uses the checker on our site now!
Best value iframe I have done yet.