Any hobby genealogy buffs in the house? What are the best on-line resources to research?

My dad brought over a bunch of old photographs he found in his closet. While he knows who some are I thought it would be fun to trace back and see if we can find out anything about the rest.

I know Ancestry charges a membership fee but are they the best of the paid ones like roots web, archives etc? What do you all think of these?

Are there any free resources that are as good?

Have any of you traced your roots and did you learn anything surprising or interesting that made you happy to have done it?

Any pointers would be appreciated.

Thanks!