Bands whose regular line-up includes an instrument not normally found in their chosen genre. Only groups who use a particular instrument regularly, so not ones who brought in a cor anglais player for one particular recording or ones where the guitarist played sitar on a couple of songs in 1968. I'm also discounting groups who are so far left field that they eschew traditional instruments altogether.
My top three are so unsurpassable that the rest of you might just as well argue about who should be at numbers 4 & 5.
5) Silver Apples (oscillators)
http://www.youtube.com/watch?v=z38hk2k8idQ4) Angel Corpus Christi (accordion)
http://www.youtube.com/watch?v=WHEcLaQkKqI3) The Thirteenth Floor Elevators (jug)
http://www.youtube.com/watch?v=TLPJFjO1Jms2) The Velvet Underground (viola)
http://www.youtube.com/watch?v=iLQzaLr1enE1) The Monks (banjo)
http://www.youtube.com/watch?v=T3fAzQzgeSc