You can send a text message from a computer through email using an SMS gateway, Google Voice or several online SMS services. Sending a text message through email requires knowing the recipient's carrier. This will determine the necessary SMS gateway. For example, to send a text message to an AT&T customer whose phone number is 123-456-7890, send an email with the message to firstname.lastname@example.org.
Any Google user can send text messages using Google Voice by navigating to google.com/voice/ and signing up for a phone number. Then press the Text button in the top left and enter a recipient phone number and message. Finally, click Send. Google Voice text messaging is free for domestic messages to the United States and Canada.
Alternatively, several websites allow users to send SMS messages for free, such as sendsmsnow.com and txt2day.com. To use either of these services navigate to the website and fill out the text message form on the home page. Txt2day.com is very similar to the email-to-SMS gateway since it requires users to know the carrier used by the recipient. Additionally, it has a field to enter a return email address. Sendsmsnow.com does not require the user to know the recipient's phone carrier; however, reply messages can only be accessed through an inbox on the web site.