I am not sure if it is as active as it used to be, but the US GenWeb project (drill down for states and counties):
http://www.usgenweb.org/
The Local History Network (again, drill down for states):
http://www.alhn.org/
Always check the state archives of the state where your ancestor was located. Missouri, for instance, is digitizing and posting death certificates, among other documents. The Illinois Archives has several databases related to military service, as well as vital statistics.
Check the local/county library websites in the localities of your ancestors. Greene County, Missouri (Springfield) and the Central Rappahannock Heritage Center (Spotsylvania, Caroline, Fredricksburg, Virginia), for examples, have extensive databases online.
And I've found plenty just by googling and binging names.