So, instead, I'll share them with you lovely people, albeit ominously shadowed....
The important thing about an omen is that it comes true. Otherwise, it's just a creepy piece of atmosphere. This means that bad omens are primarily useful only in games in which the social contract admits that the player-characters are screwed. (A bad omen in D&D, by contrast, is just a killer GM trap -- "Save versus divine fate, DC a million.") So going from "most screwed" to "least screwed," here is a flight of bad omen mechanics.
For Call of Cthulhu, the Keeper establishes certain zones -- the Martense mansion, the Dunwich forest, the Plateau of Leng -- as "Mythos Tainted." The higher the Mythos Rating, the more tainted -- Cthulhu's prison on R'lyeh, for example, would have a Mythos Rating of 100. As the Investigators move into a tainted zone -- an ominous zone -- the Investigator with the highest POW rolls a contest between his POW (or, in heroic-style games, his POWx5) and the zone's Mythos Rating. If he passes, the Investigator with the next-highest POW rolls, and so on. If nobody fails, the Keeper waits until the Investigators have passed into a still more tainted area (the cellar, the round hills at the center of the woods, the necropolis) and they contest against the higher Mythos Rating. The first Investigator to fail his contest gets a Bad Omen, which the Keeper should tailor to the area and to the Investigator's phobias and foibles. (Whippoorwills calling the unfortunate victim's name is a good default.) Here, the Keeper rolls secretly to generate an Omen Increment -- let's say 1d10, or 1d8+2. An Omen Increment of crises later, the victim will get automatic 00s on the dice for the next Omen Increment number of rolls. So if the OI was 7, upon reaching the seventh crisis (as determined by the Keeper -- most likely the seventh combat or ritual) the victim will get seven automatic 00s, and most likely suffer the horrendous fate predicted by the whippoorwills.
For Unknown Armies, the player may buy off any BOHICA (critical fumble) by accepting a Bad Omen instead. Again, the GM should tailor the specific Omen seen (mocking voices from the traffic lights, a hearse with the PC's birthday as its license plate) to the circumstances and to the player character's foibles (and to his Avatar or Adept school, for choice). The GM then rolls an Omen Increment as above, except that the player only and always receives three BOHICAs at the derived crisis. (This is the Wiccan Rede in game form, it belatedly occurs to me.)
For Vampire, the goal of the players is to get control of a situation, not survive a given crisis. The road to control lies through privileged Storyteller information, usually guarded by annoying NPCs. Hence, in this game, an omen is a trade for information -- an Oracle -- not something that fate just hands you. Before the game, the Storyteller and players work out how one goes about getting an Oracle -- crossing the fortuneteller's palm with silver, or going to the haunted tide pool under the boardwalk, or reading the entrails of a murdered priest, or whatever. Then, at any time they feel like it, the players may go seek an Oracle. For each question they ask (and the Storyteller is within her rights to restrict Oracles to one question per session), they will receive a true answer -- and one of the player characters will receive an equally true Bad Oracle, an ominous and inescapable prediction of doom. The players vote amongst themselves, before the consultation, which of them is "open to the ominous energies of this place" to get the Doom -- if one player is a real drama queen, he may even volunteer! Although this (like all Vampire mechanics) requires a good deal of Storyteller tailoring, in general, the number of dots it would normally take (in Contacts or Cult Status or whatever) to answer the Oracle question is also the scope of the evil fate.