Several months ago I wrote about LEGO creations based on Flower Thrower by street artist Banksy. Another popular work by Banksy is Girl with a Balloon that first appeared in 2002 in the South Bank neighborhood of London. Here are a number of LEGO renditions of this work, or that reference this work:
Sharon Pazner is an Israeli artist from Tel Aviv. Check out her Flickr stream to see lots of pieces using interesting materials like concrete and cut paper. Here are some works from her Concrete Games exhibition earlier this year.
(Okay, that last one had no LEGO, but it was probably my favorite.)