Humor is subjective, but these titles run the gamut from satire to cornucopias of puns, with clueless heroes and weird villains galore.

Perhaps the modern classic of SF humor, which started its own cult and became a catch-phrase. Adams' humor is distinctly out there, but then, so are his heroes. Arthur Dent's search for the meaning of Earth's existence leads him to some very strange places indeed. Nothing is sacred here, from time travel to faster-than-light spaceships, as Adams pokes fun at all the creaky and time-honored tropes of the genre. If you don't laugh, you're probably dead.

This single volume also includes:

Life, the Universe, and Everything
So Long, and Thanks for all the Fish
Mostly Harmless

Book 1 in The Magic of Xanth series:

A land of magic, tongue-in-cheek interaction and unabashed puns, that looks curiously like Florida, Xanth lies on the other side of a magical barrier from Mundania. 

Still another tongue-in-cheek series with rampant puns, heroes on farcical quests, and a really great bazaar where you can find pretty much anything, even spaghetti. Pure escapist fun with a half-trained magician's apprentice and an uncooperative demon assistant.

From the Golden Age of science fiction, the precursor to the revival managed by Star Trek, that led to Star Wars and so much more. The style is antique, the concepts seem equally so, but look around--half the military SF out there envisions future fleets patterned after what we have now. So it all looks familiar, but it's still a lot of fun, especially since this one proves that even nice guys can be a real menace.

also Kai Lung Unrolls His Mat  

Written in 1922 and re-issued by Ballantine on its 45th anniversary, Kai Lung's Golden Hours is generally acknowledged as a classic of fantasy literature, and of literature, period. Kai Lung, a wandering storyteller in ancient China, is brought before a mandarin's count on charges of treason. Like Sheherazad, he defends himself by telling stories. The book serves mostly as a frame for the stories. 

With a whole universe to yarn about and a neighborhood bar to do it in, expect mind-bending goofiness and lots of fun.

Written by a man with several years actual experience with the diplomatic corps, this are screamingly funny while ringing true to life despite the totally implausible alien worlds. Our hero, Jaime Retief, is true-blue, working for the common good despite the nefarious machinations and stupidity of his bosses. Sort of an SF James Bond, savvy, cool, and never at a loss for long.

Appropriately, this offering from a master of the genre is about a far-future reality where all the "wordwooze" is written by computers. Then one day the terminally bored human authors rise up and smash the machine, leaving "writers" to (gasp!) actually write something by themselves. Discovering themselves completely clueless, they seek help from the Silver Eggheads. You get authors' revolts and robot sex and a whole lotta laughs with this one.

First in Pratchett's zany Discworld series, wherein he has created his own private theater of the absurd. Bumbling Rincewind, whose brains may or may not have been fried by his own magic, must guide the tourist Twoflower through the ultimate flat world. No cliché is left unmined and thoroughly spiked in this delightful, and really long series:

The Light Fantastic
Equal Rites

Mort
Sourcery
Wyrd Sisters
Pyramids: A Novel of Discworld (Signed)
Guards! Guards!
Eric
Moving Pictures
Witches Abroad
Men at Arms
Reaper Man
Lords and Ladies
Small Gods
Soul Music
Maskerade
Interesting Times
Carpe Jugulum
Hogfather (Discworld, Book 20)
Feet of Clay
Jingo
The Fifth Elephant: A Novel of Discworld
The Truth
Thief of Time
Night Watch
The Last Continent: A Discworld Novel
Monstrous Regiment
Going Postal
Thud!
Making Money
Wintersmith
The Last Hero: A Discworld Fable
Unseen Academicals

For younger readers:

The Wee Free Men
A Hat Full of Sky: The Continuing Adventures of Tiffany Aching and the Wee Free Men
The Amazing Maurice and His Educated Rodents

We all love a good screw-up. They make us feel so much less inadequate. Imagine that you're an elf tasked with looking after the unicorn that links the human world with the fairy realm. Now imagine that you've discovered it's been stolen. Who you gonna call? John Justin Mallory, a down-on-his-luck Manhattan detective who manages to assimilate the fact that his employer is green and gets on with the job.

Stalking the Dragon: A Fable of Tonight
Stalking the Vampire: A Fable of Tonight  

A rather good mix of humor and history, managing to poke fun at the de Medicis in 16th century France while mixing a frustrated queen, an inept magician, and an innocent country girl with fiendish results. It starts a bit slow but stick with it; it becomes quite satisfying to watch the smug and ancient box known as The Master of All Desires meet its match at last. The author manages plot twists with the deftness of, well, a de Medici. 

A skewed collection of short stories from a master of satire, these will leave you laughing and wondering how he managed to say so much so gently. Though some of the stories are decades old, they ring as true today as when they were written.

This deliciously funny tale could only occur in the limitless universe of SF. It may be true that no good deed goes unpunished. When Captain Pausert, who wanted only to make a decent living plying the space lanes in his beat-up freighter, the Venture, frees three children from slavers, he quickly gets caught in their well-meaning attempts to be helpful. Between their, eh, interesting powers and their hot new space drive, young Pausert ends up the most hunted guy in the galaxy, discovering too late that he has fallen in with the witches of Karres.

Also classed as science fantasy.

Like the ring of power in The Lord of the Rings, the Awesome Lavratt has a mind of its own and is not easily mastered even by the likes of the lovely and determined Alanna, who is out to conquer the galaxy one planet at a time. The mind control powers of the Awesome Lavratt could help her with that, if the darn thing would only cooperate. Poor hapless Horace, in whose possession it has been for many years, has no clue what he's getting into when he meets Alanna. Wilkes pulls out all the stops for tongue-in-cheek wit, applying a dry humor all her own.