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

Cincinnati Bell
[10 Digit phone number]@gocbw.com

Cingular
[10 Digit Phone Number]@cingularme.com
[10 Digit phone number]@mobile.mycingular.com
[10 Digit phone number]@mobile.mycingular.net

Metro PCS
[10 Digit Phone Number]@MyMetroPcs.com
[10 Digit Phone Number]@metropcs.sms.us

Nextel
[10 Digit Phone Number]@messaging.nextel.com

Ntelos
[10 Digit phone number]@pcs.ntelos.net

Orange
[10 Digit phone number]@orange.net

Powertel
[10 Digit Phone Number]@ptel.net

Sprint
[10 Digit Phone Number]@messaging.sprintpcs.com

SunCom
[10 Digit Phone Number]@tms.suncom.com

T-Mobile
[10 Digit Phone Number]@tmomail.net

US CellularVirgin Mobile

[10 Digit Phone Number]@email.uscc.net

Verizon
[10 Digit Phone Number]@vtext.com

[10 Digit Phone Number]@vmobl.com