If you have your contacts mobile number and provider, you can send an email that will be a text message.
For example to send a text via email to a customer on AT&T, you would send to 1112223333@txt.att.net. See below for a known list of companies.
AT&T – cellnumber@txt.att.net
Verizon – cellnumber@vtext.com
T-Mobile – cellnumber@tmomail.net
Sprint PCS – cellnumber@messaging.sprintpcs.com
Virgin Mobile – cellnumber@vmobl.com
US Cellular – cellnumber@email.uscc.net
Nextel – cellnumber@messaging.nextel.com
Boost – cellnumber@myboostmobile.com
Alltel – cellnumber@message.alltel.com
Of course you should ask permission before you send someone a text message via email since the receiver is responsible for the cost.