The EF 85mm F/1.8 prime lens from Canon is great for portrait work. While it's possible to shoot just about anything with it, bear in mind it is a telephoto lens as well as a prime.
The EF 85mm lens is crazy sharp for a lens in its price range. No portrait photographer should be without this lens.
Where to buy: Canon EF 85mm f/1.8 USM Medium Telephoto Lens for Canon SLR Cameras