Oh Hell

Blackout
Rules for this variant. Offers play-by-email.
http://www.gamerz.net/pbmserv/blackout.html

Love That Joker
This variation adds two jokers to the deck which at as wild cards. Either joker can be played as being of higher rank than the Ace of any suit, or lower than the 2, at the player's discretion.
http://www.pagat.com/invented/love_that_joker.html

Oh Hell
Game overview from Wikipedia.
http://en.wikipedia.org/wiki/Oh_Hell

Oh Hell!
Al Okenuff's rules where you lose points if you do not make your bid.
http://www.zechiel.com/oh_hell.html

Oh Hell!
Pagat.com provides rules, variants, score sheets, related games, and links.
http://www.pagat.com/exact/ohhell.html

Oh Hell!
The rules as played at the annual Cartier Oh Hell Championship in London, England.
http://www.geocities.com/Hollywood/Academy/1450/ohhell.htm

Oh, Hell, Culver City Style
This variant plays hand size from 4 to 13 cards, high bidder names trump, point loss for missing your bid, and even bids are not allowed.
http://www.gg.caltech.edu/~jeff/games/ohhell

Romanian Whist
In this variation of Oh Hell, extra hands are played with one card and with the maximum cards, even bids are disallowed, and points are lost for missed bids.
http://www.pagat.com/exact/romwhist.html

The Rules of Oh Hell
Pratt and Whitney East Hartford Cafeteria Card Players' Club variation.
http://home.cshore.com/himes/dennis/ohrules.htm

Trixter
Known as "Stich Ansagen" in Central Europe, which roughly translates to "Predict Tricks" or "Call the Trick". Play is from 1 to 13 to 1 card hands and even bidding is allowed.
http://www.anasuya.com/trixter/