Recently I've noticed a number of new(?) ways to send an SMS from your computer so I figured I'd enumerate them here:
The latest is from today's MobileWhack: Sending a message to a phone using iChat. I just tested it with regular AOL Instant Messenger (AIM) and it works great there as wel. Simply create a new buddy with the format "+14159947917". Note the +1, so this works only for U.S. buddies, I just confirmed this. Testing the AOL thing with Erik and it's very fast, and the person receiving the message can respond to it, and it'll show up in the AOL client. Now if only I wasn't getting charged per message received here in the U.S!
The other day I posted about Teleflip.com. This allows you to send an SMS by email. Simply format the email address like this: email@example.com and it'll send off a message to the email. God knows how long this service will be around, as it seems like a great way to throw money out the window (SMSes cost real cash...) but it's cool for now.
Many carriers also provide email-to-sms gateways. For example, if you're an AT&T Wireless customer, you can send/receive emails and MMSes from your firstname.lastname@example.org. It's quite a useful service. I remember Movistar in Spain used to work that way, then they changed it so you didn't get the actual message, but a link to their web-based mail site instead.
[Later:] I just discovered that Yahoo has a whole SMS Minisite with instructions to send via Yahoo Messenger for each of the services in the U.S. There's also a full-on integration for AT&T Wireless customers, sending 'commands' via SMS to log in, send messages, etc. A little overkill I think, but interesting nevertheless. ICQ has a SMS option as well, though I don't use ICQ (too much spam).
There are also various free SMS sending services (I'm sure there are more):
And finally, there's a program you can install on your Windows PC which will send SMSes via your phone (using Bluetooth or Infrared): Microsoft SMS Sender. I've used it and it works pretty well. In a pinch you would wire it up to a server-side app (if you're that fucking stupid to be using Windows on the server) for an SMS alert system. I'm sure there's a Linux equivalent around here somewhere, I'll have to look for it.
That's all. Comments about other options welcome.