The LEGO Brick Light, has four white LEDs hidden within its studs and can be used as a bedside light or mounted on a wall (the necessary attachments are included). It measures 8cm by 8cm by 4cm, and unlike many LEGO-inspired pieces, is an official LEGO product. It is available in various colours, but it seems to be pot luck from all retailers as to which colour you will receive.
It is available to buy online from a variety of the usual retailers, including Play, priced £6.99. A cheap and cheerful stocking-filler, if ever we saw one.