Like AOL Mail, Yahoo Mail allows sending SMS text messages from a browser, even if you don't have a cell phone! It costs you nothing, but the recipients' data plan may charge for texts.
Once in your inbox, click on the dropdown arrow next to "Compose Message" and select "SMS" (or hit the keyboard shortcut of "T"), and a "Yahoo Messenger" popup will open.
Enter the full, 10-digit cell phone number in the text box, following the examples for international texts - if you've already sent texts from Yahoo, it will auto-suggest them as clickable entries.
Hit Enter to launch the message editor: type a message at the bottom (characters left from the 147 maximum are updated in real time). Optionally click on the smiley face icon to insert emoticons in your text, and hit Enter to send it.
To send SMS messages to another cell phone, click on the "+" sign button at the bottom, next the current cell number's tab - you can have multiple conversations at the same time!
Close the popup when you are done: it automatically re-opens when someone texts you back.