The most famous island is called Murano, famous for its glass works that are sold throughout Venice. Products range from glass beads to giant glass sculptures costing thousands of euros. Be cautious though, not everything being sold is "authentic" Murano glass.
Most tourists are drawn to Murano by the offer of a free boat ride to the island followed by a free tour of the glass factory. The tour usually ends with a very hard push to buy expensive glass works from their show room. My best advice for anyone visiting Venice is to SKIP Murano and head over to BURANO!