Runscope - The do it all of API monitoring and with a plethora of scripting options and pipeline hooks.  I met the founder at MonkeySpace Conference in Chicago right before he launched it and it was I could tell a seminal breakthrough for SaaS companies as it is absolutely vital for you to monitor and remedy any outages, but the resources and edge cases to consider are not what a SaaS should be concentrating on so pay up for this service while you build the best SaaS you can.

Uptime Robot - This is a free service that will run a minimal set of tests against your public API.  It works well enough for almost every situation if it doesn't cover your situation use Runscope.