I stumbled upon this wikiHow article discussing how to send SMS messages from the Internet to phones and mobile devices. After successfully testing out the email method to my iPhone on Fido’s network, I figured I’d distill the bits specific to Canadian networks.
Send SMS Messages Using Email
If you know which cellular carrier the recipient is on, then you can just send an email to their number at a specific email address:
Bell Mobility & Solo Mobile – number@txt.bell.ca
Fido – number@fido.ca
Koodo Mobile – number@msg.koodomobile.com
MTS – number@text.mtsmobility.com
President’s Choice – number@txt.bell.ca
Rogers – number@pcs.rogers.com
Sasktel – number@sms.sasktel.com
Telus Mobility – number@msg.telus.com
Virgin Mobile – number@vmobile.ca
Send SMS Messages Using a Website
You can also send a message to a mobile recipient using the carrier’s website. I’ve linked to the specific pages on Canadian carriers’ sites (the ones I could find) below:
Bell Mobility
Solo Mobile
Fido
Koodo Mobile
MTS
Rogers
Sasktel
Telus Mobility
As it turns out, there’s an exhaustive list in Wikipedia as well. Other web-based options include Facebook apps (here’s one, though I’ve never tried it, so proceed at your own risk) or a service like Send2World.
Most of these solutions require that you know which network the message recipient is on. Here’s a naive question–does anybody know an easy way to answer that question on the fly?
DarrenBarefoot.com 
You can get the network provider here using the first part of the number:
http://www.fonefinder.net/
Also, the iPhone AIM app can send SMS (they use the AOL gateways in different countries), though I found it didn’t work all the time, and usually the user couldn’t reply. Worth it though to avoid the onerous int’l SMS fees.
Very handy, thanks for that.
I’m presuming that, if a mobile number’s been ported to a different network, then you treat is as a number of the new carrier rather than the old carrier.
This in turn would mean that the fonefinder database isn’t necessarily accurate as it reports the provider that was originally issued the 1000 number block.
My niece has just moved from Melbourne (Australia) to attend Simon Fraser so this is beautifully timed.
Good helpfull, I use nice site to send sms to my friends http://www.websendsms.com , good service on good and sheap price.