I happen to be on a cell phone plan where it's free to receive text but not send it.
After some research, here are some sites that allow you to text for free. They are the cell phone company's own websites. If you know the receiver's carrier, then you can text for free.
t-mobile
https://wmg.tmomail.net/customer_site/jsp/messaging_lo.jsp?WT.srch=2&Result_Inq=answer&InqSource=TMO
verizon
https://www.vtext.com/customer_site/jsp/messaging_lo.jsp
spring
http://messaging.sprintpcs.com/textmessaging/compose;JSESSIONID_JFS=Fmkeo6qaNrIaH12d4esUKkmKP244ZEA6dH2JPfq2iU5wVARD5jjO!493563551!182801589!7060!7002
cingular
https://www.cingular.com/olam/gotoPhone.olamexecute?event=goToPhoneLandingPage&reportActionEvent=A_PHON_PHONE_LANDING_RETR
So far, not all of the text have been instantaneous, but Verizon and Sprint seem to be working well. Also, Cingular's site can only be accessed after logging into a Cingular account.
Just some random info that might be helpful...
