Here's how it works:
Open any email program or webmail site. In the To: field, put your friend's phone number (don't use any parentheses around the area code), followed by @teleflip.com. For instance, if your friend's number was 415-555-1212, you'd just enter: email@example.com
In the body of the message, type a short message, up to 160 characters. You can put something in the subject line too, but it will count towards that 160-character limit.
Hit Send. Your message will go to TeleFlip, and TeleFlip will convert it into a text message and deliver it to your friend's phone via SMS. That's all there is to it!
TeleFlip is free to use, without registration, for up to 100 messages per month. If you want to send more than that, you'll have to sign up for one of their subscription services, which start at $5 per month. It should work with any SMS-enabled phones in the U.S. and Canada.