Bloove.com - Get your phone contacts on the Bloove site, and then log in and manage them as well as send them text messages directly from the site.
ClearSMS.com - A professional SMSing service for businesses to send out mass messages to their customers to alert them about specials and sales. All messages are assigned a value in credits an you purchase more as needed.
GizmoSMS.com - GizmoSMS lets you send texts to users in over 50 countries an a slew of carriers inside each of those. While it is free to use, it does not allow space for identifying yourself except inside the body of the text.
Group2Call.com - Allows you to import your contacts, break them up into groups and send all the free texts you want to them. Also offers the chance to broadcast voice messages, but you have a limited supply of those.
Jaxtr.com - This one requires the person you wish to text to have the widget (like the one pictured above) installed on their blog or social networking profile, but if it's there, you can text the member if they live in one of any 38 countries.
OhDontForget.com - Not only can you text people, you can schedule them for later delivery.
SMSGupShup.com - A system to allow you to send one text message to a number and they will then resend it out to every member that belongs to your group, up to 5,000 people in size.
Swaggle.mobi - A service for small groups to SMS each other at once; accessible via iPhone as well as by the web.
Yahoo Mobile - Yahoo Mobile lets you send and receive phone text messages; you will not be charged for sending the SMS; but the recipient might be charged the standard messaging rates for receiving or replying.
Yellowpipe - Mainly a web host, but they happen to offer a quick and easy SMS feature when you just have to get out a text quickly. They do allow you to enter a return number.