Most of Canon’s new professional (L series) lenses introduced since mid 1999 are equipped with gaskets and rings to keep out dust and moisture. They’re not waterproof, by any means, so don’t go diving with them. But they’re much more resistant to inclement weather than Canon’s other products.

Of course, a weather-sealed lens isn’t much good unless you have a weather-sealed camera to match it. And at present only the top of the line EOS 1V, 1D, 1Ds 1D mark II, 1Ds mark II and 1D mark IIN cameras (but not the 1, 1N or 3, which have lesser sealing systems) have the same level of weather sealing - gaskets and rings around every opening, button and switch, in fact. You can use the sealed lenses with non-sealed cameras, of course, but the gasket around the lens mount will still let in water unless mated to the gasket around a sealed camera. (this rubber ring around the lens mount is, incidentally, an easy way to tell if a lens is weather sealed or not)

At time of writing the weather-resistant Canon lenses are:

16-35mm 2.8L USM
16-35mm 2.8L II USM
17-40mm 4L USM
24-70mm 2.8L USM
24-105mm 4L IS USM
70-200mm 2.8L IS USM
70-200mm 4L IS USM
28-300mm 3.5-5.6L IS USM
50mm 1.2L USM
85mm 1.2L II USM
300mm 2.8L IS USM
400mm 2.8L IS USM
400mm 4 DO IS USM
500mm 4L IS USM
600mm 4L IS USM

The mark II versions of Canon’s teleconverters, the Extender EF 1.4x II and Extender EF 2x II, also have weatherproofing. None of Canon’s more affordable lenses or cameras have weather sealing. Note also that weather-resistant zoom lenses do not have sealed glass ends - you need to put a filter on these lenses to seal out the far (non camera) end.