Every Polish region has its own folk traditions and costumes. The most popular Polish folk costumes are from Krakow - Bronowice, people in all regions in Poland would be able to recognize it. Other popular costumes are from Lowicz near Warsaw and also from Tatry Mountain area - especially Podhale. The most popular costumes for men are these from Krakow - Bronowice and also from Podhale.
Now a bit about the history of the Polish folk costumes: Folk costumes were the most trendy in the second half of XIX century when cheap and massively produced fabrics become available. Besides, peasants were not serfs anymore and their economical and political status improved, so they could afford to buy more and they wanted to show their pride by wearing traditional costumes.
Girls in Zamosc folk costumes on the photo on the right. Click inside the picture to magnify. Some costumes became more popular than others because of a surge in patriotic feelings in XIX century after Poland lost independence. For instance, Krakow's costumes became a symbol of the fight for national freedom since peasants from Krakow region took part in theKosciuszko insurrection (uprising) for freedom of Poland in 1794. We were writing about Kosciuszko insurection in the article Poniatowski in Defense of Polish Independence, II.
Quite a big number of peasants were among these who were fighting for independence. Polish peasants were called "kosynierzy" (scythebearers) since they were fighting with scythes ("kosa" = scythe). In famous Battle of Raclawice - won by Poles over Russians, some were wearing characteristic Krakow's folk costumes. In effect, folk costume from Kraków's area became a national costume. Read a story and look at some pictures from famous Panorama Raclawicka here or there.
Do people wear regional costumes in Poland in 21st century?
Not really but there are some exceptions. People in the country still wear folk costumes, in some regions more than in the other. They wear it only on special occasions - Sunday church, church processions, village feasts and weddings. Mountainers (gorale) from Tatra region, especially men, wear folk costumes on a daily basis, especially if they want to attract tourists to their business, read more about it in the article Polish Mountaineers - Folk Costumes, Tradition and History.
Here I am on a picture (left) as a girl (looking very proud) in a traditional costume from Krakow-Bronowice region. Many small girls in Kraków area and everywhere else in Poland wear folk dresses on special occasions (church and school feasts). As I mentioned before the dresses from Kraków area (where I come from) are the most popular.
In the central area of Poland between Kraków and Warsaw - Kielce, Opoczno, St. Cross (region swietokrzyski) you can still see women wearing zapaski - a form of broad shawl hung from the shoulders. Zapaska has stripes in different colors, the patterns depends on the region. On the picture here zapaska is worn by a woman in the right corner.
My grandmother had a Silesian dress whereas my grandfather had a miner's uniform since he was working in a coal-mine. Twenty years ago one could still see elderly women (babushkas) in traditional Silesian costumes (kiecki) going to the church on Sunday. Here are examples of Silesian folk dresses.
The most surviving element of a folk dress in a head kerchief. Many women of older age in a countrysite all over Poland wear kerchiefs with regional motives.
If you would like to be familiar with Polish folk art check
Polish Wycinanki Designs by Frances Drwal
I recommend Eyewitness Travel Guide to Poland (Eyewitness Travel Guides) by Teresa Czerniewics-Umer, Malgorzata Omilanowska, Jerzy S. Majewski, DK Travel Writers
This comprehensive book about Poland, showed below, may also be useful for you:
The Rough Guide to Poland by Mark Salter, Jonathan Bousfield