The QTR-1A reflectance sensor carries a single infrared LED and phototransistor pair in an inexpensive, tiny 0.5" x 0.3" module that can be mounted almost anywhere and is great for edge detection and line following applications. This sensor is sold in packs of two units.
Thumb buttons, thumb buttons, we have thumb buttons! These buttons are a nice macro size, but small and low-profile enough to plug into a prototyping PCB or breadboard. Their fast action is awesome for DIY buttons pads.