In Canada, the company I selected was Fido, which is now merged with Rogers/AT&T cellular anyway but still has lower prices.

• Naked SIM card is just $25 CDN
• 100 anytime minutes for $15 CDN, 10 cents/minute more to USA
• Number expires in about 6 months
• Here are the airtime packs
• Can’t refill with non-Canadian credit card (boo)
• Also try Rogers (now owns Fido)
• Cards hard to get, must go to Fido store or order by mail. Stores only in major malls.
• Minutes not usable outside Canada
• Can use Rogers network (a must outside Ontario) but remember to buy $5/month rogers roaming plan and turn it off when you leave. Cheaper than Rogers directly.

Note, presently $CDN = $0.81 USD, but add 15% tax in many places, making the discount more like 7% off USD.