Adafruit Electric Heating Pad - 10cm x 5cm [ADA1481]
This is hot! Literally! Apply 5-12VDC and the stainless steel fibers in this heating fabric will warm up, creating a little heating pad. On one hand, it's just a gigantic resistor. On the other hand, it's flexible, light, and can be wrapped around a project. Originally designed for portable wearable heating pads, this fabric can be used for wearables, weather balloons, diy-bio projects, thermal cycling for materials testing, etc.
The temperature reached varies with voltage. This piece is ~ 10cm long, so look in the spec sheets in the Technical Details tab for a table of voltage/current/temperature outputs. You could PWM it with a power N-channel transistor if you want to have finer temperature control.
Electric Heating Pad - 10cm x 5cm