An Air Canada "Latitude" class fare is refundable:

http://www.aircanada.com/en/news/oneway/index.html

If you book an AC ticket at aircanada.com you'll have the option to purchase a latitude fare class.

Here is how you refund the ticket:

http://www.aircanada.com/en/customercare/refunds.html

Note that latitude tickets are quite expensive, so your money will be tied up for a while. For example, YYZ <--> LHR in September:

Tango Plus (no refund): $1400
Latititude (refundable): $4400