Legal Matters
Billdo, Zappo, minty green
There are others, too, but these spring to mind most immediately.
Health and medicine
Qadgop the Mercotan
Judaica
zev_steinhardt, cmkeller, C K Dexter Haven, Arnold Winklereid
Again, there may be others, but these four have provided much enlightenment.
Business and investing
Manhattan
Thanks, hon, but you forgot my last numeral. I guess that means we’re on a first-name basis? 
Marks & Spencers undergarments
Nostradamus
Historical Musicology
Yours truly
Since they’ve not yet been claimed, I’ll offer up myself for the following categories:
Theatre
Vampire Lore
E-Commerce
What about Dragons?
I know more about ** Andrew Lloyd Webber ** than Andrew Lloyd Webber does.
I’m hurt I haven’t yet been mentioned for
Trucking
RickJay
ISO 9000 and other standards
RickJay
Baseball
RickJay
I’ll second Ringo for Geophysics.
Vintage Motorcycles
Unclvinny
If anyone has even heard of Extended Reach Drilling then I’ll offer myself. No one else knows what this, so I’m safe.
Messing up vB coding
Tapioca 
Are you boasting or complaining?
For information, past or present on NYC turn to stuyguy.
Evolution
CalMeacham
It’s actually retroviruses that I work with.
The primates are more or less incidental.
And to be honest, I’m more or less your guy for basics of virology.
Ethiopian Judaism
Kyla
You don’t know it yet, but that is, indeed, her area of expertise.
Who is an expert in landscape architecture? I need a consult.
I’ll nominate myself for a few things:
Marx Brothers lore (although Eve is the expert on Margaret Dumont)
Firesign Theatre
Graph Theory and Combinatorics (my mathematical specialties, i.e. the niches where I know more than ultrafilter)
Appreciate the nomination, whuckfistle. ;j But IMHO Sailor probably has me beat.
Also Ben is good at evolution.
Baking
Baker
Thanks for the mention. I also try to tackle questions in Zoology generally. And I’d say History of Exploration and Discovery as a sideline.
I’d say that Tamerlane does well on zoological questions too, especially for a bloodthirsty Central Asian Despot.
A few other nominations:
Linguistics: Jomo Mojo
Mountaineering: tsunamisurfer (is he still around?)
Microbiology
Third Reich and Holocaust history
Anne Frank
Michigan State lore
me!
**Romance Novels and other Superfluous non-lit **
ME