You can easily send a text message to a cell phone using email.  Most (if not all) carriers provide an SMS gateway to allow text messages to be sent.  Here's a list of carriers and their SMS gateways and the format of the recipient address

Alltel
[10 Digit Phone Number]@message.alltel.com

AT&T
[10 Digit Phone Number]@txt.att.net

Boost Mobile
[10 Digit phone number]@myboostmobile.com