Recommended lenses for a Canon 7D - Updated

I have recently taken up photography a little more seriously than before, and picked up a Canon 7D with a Canon EF-S 15-85mm f/3.5-5.6 USM IS lens. I am very happy with this lens, but would like to add to my lens collection.

What are good complementary lenses to the one I currently have?

Update:
It was correctly pointed out that I should have stated the nature of my photography. I mainly do portraits, landscapes, and once in a while sports shoots.