Google Calendar includes an option to send reminders via SMS -- but with a little tweaking, you can use it as a means of getting free SMS messages from your friends and family.
Reader Chris L writes in with this suggestion:
Create a publicly editable calendar, and set SMS notifications for it. A friend simply has to add an event a few minutes into the future and then it will send you an SMS.
Frankly, I thought this sounded unlikely when I read it, but on testing it works fine. (Doubtless the fact that in Google's home US market SMS charges are paid by the receiver led to the creation of this loophole.)
To set this up, create a new calendar within Google and tick the 'Make this calendar public' option. Add any friends who you want to use the service in the 'Share with specific people' section, and set the option to allow them to make changes. Then click on the Settings options, click on Notifications, and select SMS notifications. This screen will also let you set up your own phone number to receive notifications if you haven't done that in the past (Google sends a confirmation via SMS which you'll need to register).
Potentially, if everyone you knew set up a similar calendar, you could all send messages without incurring any costs. Of course, that presumes you're near a PC where you can enter the relevant details, but it's a clever hack for the cost-conscious. Thanks Chris!