The Code Fairy - Eats semicolons from source code, causing working computer software to stop working
The Grim Reaper - A common anthropomorphism of Death
Hotei - A Laughing Buddha
Invisible pink unicorn - From alt.atheism, a fictional deity said to be the bane of theists everywhere.
Lady Luck - Invoked by gamblers
Mother Nature - Manifestation of the Earth and female power
The Parking Fairy - Responsible for finding parking spaces.
Santa Claus - Originally Nicholas of Myra, but later turned into Father Christmas.
The Tooth Fairy - Fairy spirit of growing up and dental hygine.
The Traffic Light Fairy - Changes the color of traffic lights
The Gremlins (of given small objects) - The evil spirits that spirit away pens, paper clips, files, and keys.
The Holidays - The various immortal spirits that express and personify the given holidays (Cupid, Lepricahn, Easter Bunny, Spirit of Freedom, Santa Claus, and The Aging Baby, among others).
Additional Ideas (10)
Theater Gnomes - These creatures live under the rows of seats, and take or move any tools or other objects left out over night.
The Boogyman - also called the Monster Under the Bed or the Monster in the Closet. Embodiment of childhood fear of the unknown and the dark. Sometimes a good deal more real than people like to admit.
Not Me - an invisisble, often malicious spirit that does quite a bit, judging by the amount of blame laid at his feet.
Murphy - originator of "Murphy's Law" (What can go wrong WILL go wrong. Corrollary - It'll generally happen at the worst possible moment.)
Finagle later came along and condensed this to "Anything that can go wrong, will." and then instantly became victim of his own law as everyone credited it to Murphy.
A malevolent, spiteful entity whose sole existence, it seems, is to be the cause of those mysterious, inexplicable slowdowns, traffic jams, and other such events which make commuting in any large city an absolutely pain in the neck. Often seems to single out individual drivers and plague them for days at a time, then just as mysteriously turn its attention elsewhere.
Fnord, Spirit of Conspiracy
Often invoked by conspiracy theorists, Discordians, and others of similarly fringe nature, Fnord is the essence of the inexplicable when trying to explain it. Fnord is often blamed for almost anything, if a conspiracy can somehow be blamed for the event, however implausibly. Originally an innocent joke by the writers of the Principia Discordia, Fnord has taken on a dramatic life of its own in the lunatic fringe.
Breaking down all barriers to communication, Trillian is the Spirit of True Communication. Its powers are only begining. Eventually, as Trillian grows in power, the net will facilitate a higher degree of communication.. and increase Humanities (and AIs) ability to work as one.
For now, Trillian is the spirit of chat. It inspires people to write chat programs and add ons. It can ward against Gremils and improve latency of all blessed chat programs.
They tended to be more serious than we.
These malevolent little cold-weather demons demand offerings of chapstick or other lip balms. If they are not appeased, they attack the lips, often until they crack and bleed.
Bugster is a higher spirit of chaos. It loves to infuse complex systems and new projects, where it creates large numbers of bugs, frustrating the developing, testing and maintenance departments. Some claim it is a mere aspect of Murphy, but that is not true. Where Murphy is perfectly content to produce the same bugs or hit in the most obvious places for extra annoyance, Bugster likes to be creative, often choosing the most obscures corners for the defects to lurch from. He never repeats himself, though the bugs may be reproducible.
It is possible to attract him with the proper rituals (or numerous meetings) or lure elsewhere. To banish him is very hard.
The Birds and the Bees
Common name for the spirits that watch over a child as he or she transitions into adulthood.
A malignant water demon that exists only to drag sailors down to their deaths on the open ocean.