Canon EF-S 17-55mm f/2.8 IS USM Lens for Canon DSLR Cameras My walk-around. It's not technically an L but has L lens quality. The wide aperture and IS make it great for low light.
Canon EF 70-200mm f/4 L IS USM Lens for Canon Digital SLR Cameras The 4-stop IS and incredibly sharp optics are just fantastic. Definitely my favorite lens. Used mostly for portrait work.
Canon EF 100mm f/2.8 Macro USM Fixed Lens for Canon SLR Cameras My only non-IS lens, but does what it's supposed to: open up a whole new world.