all NEXUS phones are unlocked.
Most Blackberry's can be unlocked with free online calculators (i.e. freemyblackberry.com)
most Nokia's can be unlocked if you download their support software
and Androids can be unlocked as well.
Prices for unlocks vary from $3 to $50, depending on if you go online or walk into Pacific Mall and haggle with one of the many phone stores there.
Some providers will provide you with an unlock code if 1. you bought the phone outright and 2. have at least 3 months of service with them (i.e. Wind Mobile, etc). You just have to call and ask. With Bell, I called and said I'm leaving the country and want to use the phone where Bell doesn't have coverage, so they gave me an unlock code.
YMMV.