Library clips has a list.
Groups SMS is in vogue at the moment as there are lots of services popping up. These services all have the main purpose, sending a group of people or your contacts a message they can access on their phone, and being able to reply to the group or senderbut these services do differ in the details.
Some have a web and mobile web presence (even a social network), some you can send text via a webform or email, and even send and receive text by RSS, email or IM.
Some of these services are more leader to group (one to many) one way communication, whereas some are many to many, eg. one to group (or contacts) or group to group (or contacts) SMSing, and replying.
Some have group voicemailing, and there are a few that have a Text-In information service.