I compromised on some of the prices to get better values.
Bend / flex sensor http://www.imagesco.com/sensors/flex-sensor.html The $4.00 out-of-spec sensor would often suffice.
Fabric/stretch sensor http://www.happystim-usa.com/catalog/i69.html
This was almost the only source of small quantities of conductive rubber I could find. Making a stretch sensor is as easy as attaching terminals to the ends of a strip of conductive elastomer, so that’s what I looked for. Apparently anti-esd rubber bands have resistance in the megohm range, which is not as useful. One $8.00 sheet could make dozens of sensors.
Electroluminescent “wire” (EL-wire) http://www.cooltronix.com/zencart/index.php?main_page=product_info&cPath=68&products_id=185
Alcohol sensor http://www.junetrading.com/product_info.php?products_id=5593
This one allows precise position control: http://www.herbach.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=HAR&Product_Code=TM02MTR4562
Fiber optic cable http://www.meci.com/product_info.php/products_id/6400057
Bag of small potentiometers: http://www.elexp.com/cmp_grab.htm
Single tiny potentiometer: http://www.mouser.com/search/ProductDetail.aspx?R=652-3364W-1-103E
Conductive fabric: http://www.blockemf.com/catalog/product_info.php?products_id=5353 Cheap tacky metallized craft fabric would probably work for some applications.
Thermochromic paint/dye http://www.rapidonline.com/productinfo.aspx?tier1URL=Educational-Products&tier2URL=Graphics-Art-Design&tier3URL=Materials&tier4URL=Thermochromic-pigment&moduleno=73050 Nice minimum quantity, and you can easily choose the intensity of the tone by adding the appropriate amount of medium.
Nitinol / shape memory alloy wire http://www.amazon.com/Nitinol-Wire-SuperElastic-Titanium-Straight/dp/B0013891KI/ref=pd_sbs_indust_2
Conductive paint http://www.trainaidsa.com/shop-conductives.shtml Is the RF shielding stuff conductive enough to handle useful current? If so, it’s a better value if you need a lot.
Neodymium magnets http://www.dealextreme.com/details.dx/sku.4248 I ordered some from here and they’re the real deal.
Copper tape / copper foil
Minimum price for a roll: http://www.acehardwareoutlet.com/(5eh5ye55nbzwhs55pwdljebf)/productdetails.aspx?sku=998060152&source=GoogleBase
$0.25 and detects on two axes. Probably not very reliable.
These are sweet.
What can’t you use with an Arduino? With a big enough semiconductor switch, you could control a Transcranial Magnetic Stimulation coil.