Any recommendations for web apps for sending bulk SMS to several mobile numbers?
Clickatell and Commzgate support bulk messages - both have easy-to-use web APIs as well. Both these services are quite expensive though.
If most of the SMSes you are sending are local, you can setup an SMS gateway (ozeki, visualtron etc), a GSM modem and send SMSes using a purchased SIM card. Remember to test using a prepaid SIM to avoid running up a large phone bill in case the automated system malfunctions.
I've used TextAnywhere for years. (This is a UK service I think)
you have to check 2-WaySMS
Commzgate also has an free SMS gateway server that you can download and send SMS with GSM modems