I believe that Canada currently allows dual citizenship with a variety of countries. I believe this to be the case since I read recently about a push in government to revoke passports for Canadians living abroad. During Israel's attack on Lebanon this summer, the Canadian government got thousands of people with dual Canadian/Syrian or Canadian/Lebanese citizenship out of the line of fire.
I have some friends who have dual Canadian/U.S. citizenship so it would seem that that is also possible. I also think that dual citizenship is possible among different Commonwealth nations, and in fact is easier than in most other cases -- there are lots of dual Canadian/Brits around here.
I am personally interested in dual citizenship; unfortunately I don't know the exact regs for any of the countries I've listed. In the case of Canada you could find out about it at
www.cic.gc.ca (CIC = Citizenship and Immigration Canada).