Canon's 'nifty fifty': the little lens that could
The Canon EF 50mm f/1.8 lens is small, lightweight and versatile -- the necessary first expansion beyond the kit lens when adding to your Rebel's repertoire. For its price, the beloved 'nifty fifty' is quality glass -- it's fast for low light, and its wide-open aperture gives portraitists the shallow depth of field and blurred background that makes magic.
If you'd like to take better portraits, the nifty fifty is a great place to start. It's a fixed focal length, which means it doesn't zoom in and out -- you'll have to change positions in order to compose your shot. But to get a variable focal length (zooming) lens as fast as this, you'd have to spend several hundred dollars.
The Canon EF 50mm f/1.8: for the aspiring portraitist, it's a must-have.