Ok, now that y'all have flashed around the "L" word for a while (both for L lenses and Leica) I will throw in an absolutely indispensable bit of kit for me that is third-party, unsexy, slow and dirt cheap...
Let's hear it for my lowly Tamron AF70-300mm F/4-5.6 Di LD Macro 1:2 purchased as part of a D80 kit.
I originally planned on using the lens until I could afford to upgrade, then sell it on
eBay for $50. You see them all the time brand new for around $100. I would rather part with fingers on my hand than this lens, let me tell you why.
It is absolutely sharp as a tack through most of the range (tailing off a bit at 300mm), weighs absolutely nothing (less than a pound), zooms well, has a good 1:2 macro setting, and did I mention it was sharp as a tack? It also has a lens hood that doesn't shadow the built in flash.
This is my carry around lens for my zoo work wherever I can use it... letting me keep the BigMa in the bag (in the trunk of my car) in many situations.
The lens is pretty well disposable at this cost, so I don't have to worry about expensive glass getting banged around in a crowd... and it is light enough to carry all day (sometimes I am shooting for 10 hours a day, walking many, many miles).
Unsexy, slow and cheap... but I wouldn't get rid of it for anything.