Just like Mannclann said, I don't know anything about Canadian tax laws, but I'm sure they're not too different than here in the US.
You can deduct anything (within reason) that pertains to the operation of your business.
Some of the things I deducted on last years taxes (as they pop into my head in no order):
Office rent
Computer
Internet Connection
Webhosting
Software
Mileage for work-related reasons
Dining / Entertained for work-related reasons
Software / similar resources (training programs, etc.)
Outsourced expenses
Ummm... it's 1AM here and I've had a long day so that is about all I can think of. In short - you can deduct nearly anything that is business related, but also as Mannclann said, consult a certified accountant regarding the specifics.