Hodgdon Data Center is one of my most used manuals. I've got several printed manuals as well but the Hodgdon site has a lot of data and you can't beat the pricetag either!
They cover Hodgdon, IMR, and the Winchester powders.
It is good to have a paper manual available though too.
If you are just reloading for one or two calibers, check out the LoadBooks offerings. They have caliber-specific data compiled from most of the major manufacturers in one book.
You can find em on ebay every now and then too.