Listmania!
Great Modern Cartoons
By an Amazon.com customer
Evil Chickens Don't Kiss: Goats: Volume IIIEvil Chickens Don't Kiss: Goats: Volume III by Jonathan Rosenberg
Goats (http://www.goats.com) is probably the best cartoon strip that's never been syndicated in newspapers.
Flashbacks: Twenty-Five Years of DoonesburyFlashbacks: Twenty-Five Years of Doonesbury by G. B. Trudeau
Buy used from: $0.70
Unfortunately most of the original books are out of print, but this is probably the best Trudeau collection readily available.
Abs Of Steel, Buns Of Cinnamon: A Cathy CollectionAbs Of Steel, Buns Of Cinnamon: A Cathy Collection by Cathy Guisewite
Buy new: $9.95 / Used from: $0.01
Yeah, I like "Cathy." Wanna make something of it? Growing up without sisters, it was a great way to gain insight into the female mind.
Bloom County Babylon: Five Years of Basic NaughtinessBloom County Babylon: Five Years of Basic Naughtiness by Berkeley Breathed
Buy new: $17.09 / Used from: $0.01
Again, most of the originals are out of print; this collection spans the best Berke Breathed had to offer, though.
Get Down!! Dog CartoonsGet Down!! Dog Cartoons by John Callahan
Buy used from: $0.01
Perhaps one of the most twisted individuals ever to put pen to paper.
When We Can't See The Forest For The BushesWhen We Can't See The Forest For The Bushes by Pat Oliphant
Buy new: $14.95 / Used from: $3.63
Not the best-known political cartoonist, but certainly one of the most daring.
The Indispensable Calvin And HobbesThe Indispensable Calvin And Hobbes by Bill Watterson
Buy new: $11.55 / Used from: $0.33
Like Bloom County, Calvin & Hobbes was short-lived enough to go out at the top of its game.
Peanuts TreasuryPeanuts Treasury by Charles M. Schulz
Buy used from: $0.01
To my mind, Shultz created the first modern comic strip, and probably still the best. We miss you, Chuck.
Another Day In Cubicle Paradise: A Dilbert BookAnother Day In Cubicle Paradise: A Dilbert Book by Scott Adams
Buy new: $9.34 / Used from: $0.27
Sometimes I think Dilbert would be better if Scott Adams hadn't quit his day job, but it's still pretty funny to be able to recognize your own workplace on every page.
Two Guys Fooling Around with the MoonTwo Guys Fooling Around with the Moon by B. Kliban
Buy new: $5.95 / Used from: $1.41
I respect anybody who has the guts to put the bizarre surrealism in his head down on paper for the world to see.
The Chickens Are RestlessThe Chickens Are Restless by Gary Larson
Buy new: $10.39 / Used from: $0.01
Gary's surrealism was just a little bit more harmless than B. Kliban's... which made it more accessible and more popular.
Family Business:  For Better Or For Worse CollectionFamily Business: For Better Or For Worse Collection by Lynn Johnston
Buy used from: $1.84
The only popular comic strip I know of where characters age in (more or less) real time.
Boondocks: Because I Know You Don't Read The NewspaperBoondocks: Because I Know You Don't Read The Newspaper by Aaron McGruder
Buy new: $10.39 / Used from: $2.13
Unafraid and unapologetically controversial, some of the funniest strips of the last few years are McGruder's.
The I Hate Unicorns BookThe I Hate Unicorns Book
Buy used from: $2.99
Unicorns were huge with teenage girls in the 1980s; this book skewers everything about them, and it's beautiful. Take my word, not the lack of info on Amazon's page.
I Try to Take One Day at a Time, but Sometimes Several Days Attack Me at OnceI Try to Take One Day at a Time, but Sometimes Several Days Attack Me at Once by Ashleigh Brilliant
Buy used from: $0.07
It's hard to believe this is difficult to obtain. Brilliant's humor is warped, but subtly - it sneaks up on you.