Details at Bell Mobility

Additional $0.05/MB
One time activation fee: $35
Paper billing fee: $2

Data Stick (with 3 year term): Free

Cost: $65 / month

Details at Mobilicity

Data Stick: $79.99

Cost: $40 / month

Details at Rogers

Additional $0.05/MB
One time activation fee: $35

Data Stick (with 3 year term): Free

Cost: $65 / month

Details at Telus

Additional $0.05/MB
One time activation fee: $35

Data Stick (with 1 year term): Free

Cost: $65 / month

Details at Wind Mobile

Data Stick: Free

Cost: $35 / month

Fido:
http://websms.fido.page.ca/2way/

Koodo Mobile:
http://koodomobile.com/en/on/sendamessage.shtml

MTS Mobility:
http://text.mts.net/

Rogers Wireless:
http://www.rogers.com/web/content/wireless-text/send_message

Telus Mobility:
http://www.telusmobility.com/en/ON/send_message/index.shtml

* Fido and Rogers Wireless offer two-way text messaging online, so you can actually get a reply back from your recipient.