GIMP is pretty decent feature-wise i think.. it is indeed free.
picasa is pretty skimpy in features.
for B&W, i took a quick look in google for you with regards to GIMP and B&W conversion.
it says that there is a channel mixer function:
http://www.gimp.org/tutorials/Color2BW/
as far as i'm concerned, that is sufficient for great B&W conversion. everything else additional with all the gimmicky plug-ins, etc.. that's just convenience, you can do it with curves and simple generalised Burn&Dodge.
if none of these terms like "curves", "burn", "dodge", "channel mixer" make sense, my advice is to read up more on digital B&W conversion, and basic post processing.