Adafruit's conductive thread is 23 meters of 2 ply thread, a little thicker than every day polyester or cotton thread but still thin enough to be sewn by hand in medium-eye needles or with a sewing machine that can handle 'heavy' thread. Because it is strong and smooth, its ideal for any wearable/e-textile project. It also has fairly low resistivity, 16 ohms per foot so you can use it to drive LEDs and other electronic components that use under ~50mA.
Because it is made of stainless steel fibers, it will not oxidize like silver does: your projects will not 'stop working' because of oxidation after a few months and its safe to wash.
Measures 0.2mm thick, 2 ply thread, 1.3 ohm per inch