go here to the resources section There are not many free ones there but it is a start.
Just google Bed & Breakfast Directories and I am sure many will show up! See any you like that are free...feel free to sign up....though I doubt any of them will do you any good.
if it is free b&b advertising you are speaking of, best for me was reciprocal linking with local businesses that make sense.
for my b&b, this was a local ferry service that took people across to an island for day trips and stayovers; a kayak rental and tour service; a general store (it was a destination); a couple destination restaurants, shops and museums; the town page that offered a free listing with no link back required; and a few like-minded b&b's in other areas where folks tended to be heading to or coming from.
also, sites like this one!
Here is a list I have compiled...a few are specific to Michigan, but most are not. The links are directly to each site's submission page.
And I second SS's suggestion. Some of our best web traffic comes from local attractions and restaurants...and most are happy to do it for your link to them.