Send a text from email:

It is sometimes convenient to send a text message from your email account rather than from your phone. In order to do this, the recipient's cell phone number must be converted to an "email" format. Here's how:

(from http://www.popularmechanics.com/technology/how-to/4318204)

All major mobile-phone carriers have e-mail addresses that feed into your SMS inbox (see our diagram for specific addresses). If you have a smartphone and an all-you-can-eat data plan, you can use your phone's e-mail client to send as many texts as you want for no additional cost. Of course, this method has its limitations: In order to determine what address to send your message to, you first need to know what cellular provider your friends have. I suggest asking your most-texted contacts who their providers are. With that information, you can program the proper e-mail address next to their names in your phone's address book.

Easier yet: Send your text message over AOL Instant Messenger (versions of which exist for most mobile phones). Instead of sending your message to a screen name, send it to "+1" followed by the area code and phone number. So if your friend's number is 212-555-5555, sending an IM to "+12125555555" will ensure it pops up on his cellphone screen. If your phone has Web access, you can also send free texts through sites such as txtdrop.com.

Of course, you still have to pay to receive text messages from other people. If somebody replies to a text message that you sent as an IM or e-mail, his reply will come to you in the format you initially sent it in, and reading it won't cost you a thing. You can cut your bill even further by asking your most-texted friends to program your e-mail address into their address books. Even if they insist on using SMS, sending a text message to your e-mail address will cause it to appear in your e-mail inbox. And as long as you have an e-mail program on your phone, it will be just as easy to retrieve.

Your target's e-mail address depends on his mobile provider:

Provider	Format
Sprint	phonenumber@messaging.sprintpcs.com
Verizon	phonenumber@vtext.com
T-Mobile	phonenumber@tmomail.net
AT&T	phonenumber@txt.att.net
AIM	+1phonenumber

So, how do you know what your friend or coworker's service provider is? You go to this site:
http://www.whitepages.com/carrier_lookup?