At least two artists use this name:
1) Willem is a Dutch rapper and TMF VJ. Together with Big2 (Twan) he forms the Dutch HipHop duo The Opposites.
2) Willem Verstraaten is a 20 year old singer from Geldrop (The Netherlands).
He participated in the second edition of the Dutch 'So You Wannabe a Popstar'. During the program, he became very popular among the female fans, however he did not win the program, but became third after Tessa and Maaike.

Read more about Willem on Last.fm.