Anyone with a Google account can send text messages from the Web for free using Google Voice. Navigate to Google.com/Voice and sign up for a virtual phone number if necessary. Once done, click the Text button on the top left of the page. Enter the recipient's phone number, the desired message, and press send. This can be used to send messages to up to five recipients.
Another way to send SMS messages from the web is using email and SMS gateways. This requires knowing the carrier the recipient uses. For example, to email as an SMS to an AT&T customer with the number 123-456-7890 , address the email to email@example.com. A fairly comprehensive list of SMS gateways is available at emailtextmessages.com.
For the majority of users, the tools offered by Google Voice or email to SMS gateways should be sufficient. Nonetheless, there are a number of paid Web based SMS services that may be preferable for heavy users and businesses. Bloove.com, ClearSMS.com, and GizmoSMS.com all offer Web-based SMS tools. Blooze.com allows users to import and manage their contacts as well as send text messages. ClearSMS.com is an ideal service for mass messaging. GismoSMS.com offers Web based messaging to a variety of carriers in over 50 countries.