The latest issue of the Institute for Historical Research points to a couple of rich online sources.
The History of Parliament online – pretty much what it says on the tin.
And the Connected Histories, covering 1500-1900. (Not all of the databases linked to are free, but all give at least a snippet, giving a sense of what’s there.)
Feeling very old when I think I can actually remember when the first CD journals arrived in the university library and I discovered the joys of full-text search!