One of the necessary items on our Christmas tree every year are strings of bubble lights. These glowing little guys were being manufactured in England as early in the 1940s; they graced my grandma's Christmas tree, my parents' tree, and have been on the Brick family tree ever since our first year. (We were married the day after Christmas 1981, so had to wait a whole year before our own festivities.)
For a general look at bubble lights, visit here. They're a little difficult to find, but worth it. Sitting by the Christmas tree, watching the lights glowing against the iced window...there's nothing more cheerful on a snowy night.
(ill. courtesy of Wikipedia)