I want to add the ability to send reminders to people via email and SMS for specific events that they have signed up for on a web application that I am building.  The email part is not difficult, but I am wondering where to find a good solution for sending SMS messages.It would also be a plus if this solution allowed two-way SMS communication with my web application so that people would be able to reply with a CONFIRM or CANCEL type of a message.Has anyone implemented something like this? Does anyone know of good tools out there?EDIT: I am realizing that this is more of a lots of ways to skin this cat type of question and so I changed it to community wiki.
How to add SMS text messaging functionality to my website?
notification;sms;web services
I've had to edit this answer a few times as the services I initially recommended have gone out of business. I believe that the new industry leader is Twilio, they have an excellent API and reasonable prices. Some alternatives worth considering are Tropo, Plivo, and Nexmo. I've been using IMified for a little while to do stuff similar to that and I've found it to work well. It allows you to receive SMS (or Twitter and IM messages) as HTTP POSTs and your web server's response is sent as a reply. They also offer a web service that can be used to initiate conversation and send messages.While I was looking for other options I came across Zeep Mobile(Note: Zeep Mobile will no longer be in business as of 8/1/2013) which has an API for sending and receiving SMS. It is either free and ad supported or available for a montly fee. I have not tried it, but it looks quite interesting.You can also use an SMS Gateway like ClickATell.Best of luck!EDIT: I found a question on StackOverflow that has lots more suggestions, some may be outdated.