OK, here's some of them. They aren't in any particular order, just the order I dig the links out in. :-)

Rokiczanka They only have a small repertoire, but what they have is quite good. You can view the website in either Polish or English, and they have videos on YouTube as well.

Poloniamusic A very useful resource, especially for finding lyrics.

Mazowsze Another good group, they also have a fair number of videos on YouTube.

Hej Sokoły (Hey, falcons) Probably one of the most famous Polish folk songs, also a Ukrainian song (with different lyrics), and my favourite recording of one of my favourite songs. :-)

Stiep da stiep krugom (Steppe all around) OK, this song is actually a Russian yamshchik song, but is well worth listening to. Słowianki is a Polish group, however, and has a bunch of other good songs that actually are Polish, some of which will follow. :-)

Czerwone jagody (Red berries) Another of my personal favourites, again, sung by Słowianki. :-)

W żłobie leży (In the manger) A Christmas carol. If you know Infant Holy, Infant Lowly, this was the original. A pretty good recording, I haven't researched to find out if any of Irena Santor's others are any good.

A mazurka Słowianki again, this time dancing a mazurka.

W moim ogródeczku (In my garden) A Rokiczanka recording of another favourite of mine, but with pictures of traditional dress from around the country instead of their video.

To i hola I don't know what the name of this one means. Mazowsze singing another favourite of mine. Rokiczanka also sings this one.

Cyt, cyt I think this one's name means "Chirp, chirp!", but I'm not positive. Mazowsze again.

Varshawianka (Polish Warszawianka) This is another Russian song, this time sung by the Red Army Choir, but it's also a Polish song, though I haven't looked for Polish recordings. Wikipedia Warszawianka page

Karolinka I don't remember who this is, but it's a pretty good recording.

More Mazowsze I don't know what these are called, but they're also pretty good. :-)

Mazowsze concert A whole concert of Mazowsze. :-)

Mazowsze in Russia Another Mazowsze concert, in Russia.

Ej, przeleciał ptaszek Another Mazowsze song that I don't know the meaning of the name, but still good. :-)

I've got more, and we haven't even got to one of my favourite groups, but I'm running low on time, so I'll add them later. :-)