If you need to send text messages to people's phones from your computer and don't want to have to remember much beyond their phone number, you should check out Teleflip. Teleflip works like this:
- Open an e-mail client.
- Put the address as <10 digit phone number>@teleflip.com.
- Type your message.
- Press send.
There's no charge for the Teleflip service and the site claims that it works pretty much everywhere in North America, regardless of the cell phone carrier. Note that text messaging charges can apply to the people that receive the message, text messaging must be active on the phone your are sending to for people to receive a message, and there is a character limit as the message comes through as a SMS.