Shrdlu is a game of finding a five- or six-letter word that fits into a set of randomly-dealt constraints. It's a print-and-play deck of fifteen cards.
Shuffle the deck and deal out a row of five cards. (If one of those cards has the "deal out a sixth card" instruction, deal out a sixth card.)
An example spread would be:
Players then race to think of a word where each letter of the word obeys the constraint on the card in the corresponding position, with asterisks allowing any letter.
(So in the example above: a word that starts with E, T, A, O, I or N; whose third letter only appears once; and whose fifth letter is alphabetically later than its fourth. The second and fourth letters can be any letter, and the word must have five letters in all. A valid answer would be NIGHT.)
The first player to call out a valid word wins the round.
If a player believes that nobody will be able to find a word during a round, they may call "shrdlu!", if nobody else has done so - their opponents then have thirty seconds to find a word. If anybody does, that player wins the round as normal; if nobody can find a word, the shrdlu caller wins that round instead. (A player who calls shrdlu may not also call a word for that round.)
The winner of a round takes the leftmost card of the word and keeps it face-down in front of them for scoring.
After eleven rounds (when there are no longer enough cards to deal out another round), the game ends and the player who took the most face-down cards wins. If tied, the tied players reshuffle the deck and play a final tiebreaker round.
Note: The "repeat any of the final 3 letters" card cannot choose to repeat itself, if it is dealt into one of those positions.
Bearing in mind the interchangeable asterisks and the possibility of dealing a sixth card, and the unusual final round possibility of "deal a sixth" plus four asterisks, there are (if I've got this right) 306,201 distinct card spreads possible in the game of Shrdlu.
Of these, 4,051 have no dictionary solution (at least according to the Collins Scrabble dictionary), meaning that 98.7% of card deals have at least one possible answer. Although those answers may be obscure dictionary words that nobody at the table will know.
You can use the Shrdlu Card Server to try the game out, or to play the game around a single phone screen if you don't have a deck printed out.
You can play the game remotely on playingcards.io by importing the shrdlu.pcio game file into a room. Players should take cards into their hands when they win a round, and must hit the "Deal a 6th card" button manually if the "deal out a sixth" card appears in the spread.
There's also a daily Shrdlu deal Twitter bot running at @dailyshrdlu, which automatically filters out the 1.32% of deals with no dictionary-recognised answer.