We recently purchased some silhouettes from an estate sale and found these two sites to be invaluable in researching them.

Peggy McClard  and Silhouettes Blog Spot

For asian Antiquities Research we always check Here First.

For research of Porcelain Marks try www.dictionaryofmarks.com it's as subscription service but worth the money if you will use it often.

For art work try www.askart.com Also a subscription service but worth the money if you will use it regularly.

Researching Clocks? try www.antiqueclockpriceguide.com

And as always www.kovels.com has great information.