Does anybody know anything about "Hopkins & Allen" black powder rifles?
History, collectability, etc?

It depends on what you really have from Hopkins & Allen. Is it an original rifle or a reproduction? So more information is needed to really answer your question. Take care...
Oberndorf

I don't know how old it is. It was given to me from my brother. His boss bought it for him
as an anniversary reward (10 yrs. employment.) From a dealer in Claremore, OK.
i received it several years ago when he (brother) purchased an in-line CVA.
Here is all I know,
50 cal
St Louis style percussion (like a Hawken)
octagon barrel (Blonde stock)
double set triggers
Made in Spain (stamped)
It looks like at one time it was a 45 cal, but .50 is stamped over it
CVA, and Traditions parts fit it (nipples,etc.)
Wood ramrod is 5/16 inch. (guide holes)

Sounds like you have a reproduction rifle. Shoot it and enjoy it. There is limitied collector value for these rifles. All the best...
Gil

If not mistaken I believe the original H&A muzzleloaders were underhammers.

http://www.gunbroker.com/Auction/ViewItem.asp?Item=89655883

Numrich bought up most of the stock in the '60's I think, which this example appears to be one of.