They're fairly mainstream now, but everyone should check out The Arcade Fire, Broken Social Scene, Bright Eyes, Spoon, Interpol and Metric.
For less well-known bands, Air Traffic, Robbers On High Street, Okkervil River, The Mountain Goats, The !@#$% Fairies, The Dresden Dolls, Shearwater, Belle & Sebastian, Headlights, Lodger, Maths and Physics Club, The Meeting Places, Suburban Kids With Biblical Names, Mew, The Cribs, The Good Shoes and Sibiria all come highly reccomended.