Well, someone beat me to www.familysearch.org - but that's a treasure trove of sources that are free. They've definitely had some issues both with indexes that need a little work, and source photos that seem to never load properly, but overall it's a very good site. And the member trees, while not always perfect, can give you clues that may lead to breakthroughs, so I always check the trees, ancestral, and pedigree resources to see if there's something interesting that I can use as a starter to follow up on.
Along those same lines, I'm always looking for a way to generate ideas about which way to look for ancestors. While they're not perfect, I've been known to use rootsweb and geni as ways to see what other research has been done and whether or not they've been kind enough (rarely, I know, but sometimes it does happen) to supply some real sources for their information. 95% of the time, the source is non-existent or something like "ancestry ft", but once in awhile they'll mention a book or actual sources that can be verified. So I try not to count them out.
Being involved with Notables, I often find myself spending time with peripheral sources, like ethnicelebs, Wikipedia, and IMDB. They often have little tidbits or starter information (birth/death date/place, marriages, sometimes parents, real name, etc.).
Find A Grave and Billion Graves are always good research sites, although I've found you can't always take for granted that everything is completely accurate. Even if the record is properly indexed, I've found at least once where the picture said one thing - and the index said another. I try to always recheck my data, just to be on the safe side.
And in the US while at a public library, access to Ancestry is FREE - so if I'm in a pinch and need some Ancestry resources, I set aside time to make a trip to the library and make the most out of what resources I can gather that are unique to this site. You have to be on their WiFi to get the free access and have a library card, but that's usually not too difficult to obtain. Or you can sometimes use a guest account on one of the library computers.
I'm sure there are a bazillion more, but those are the ones I use the most.
