well,
http://rltiles.sourceforge.net/ are free, as long as you provide their information any time you use them (which i have forgotten to do...bad me...)
and those provide quite a lot ot work from.
but i would not use anything else without contacting the owner/developer/artist/whatever.
i downloaded lots of sprite sheets, but i'm afraid to use them
(as krice said freeware does not equal "steal my assets"...but if you ask and give credit, lots of people are ok with it)
i'm using a combination of the david gervais tiles , and a few from
http://rltiles.sourceforge.net/