Hackmaster (also spelled HackMaster) is a roleplaying game produced by Kenzer & Co, which first appeared as a thinly veiled parody of Advanced Dungeons & Dragons in their flagship gaming comic, Knights of the Dinner Table. In 2001, Kenzer bought the licensing rights for AD&D from Wizards of the Coast as part of a court settlement about Knights of the Dinner Table and promptly released Hackmaster as a real game, which continues to this day.
Hackmaster is, at its core, a retroclone - or at least a parody thereof. See, whereas your standard retroclone is all about celebrating the grognard vision, Hackmaster throws Old School Roleplaying a cheerful pair of double-middle-fingers, lampooning the fact that despite what rosy memories claim, Advanced Dungeons & Dragons was an absolute mess of systems, sub-systems and sub-sub-systems that Dungeon Masters chopped, swapped, and rearranged into Frankenstein's Monster affairs that were ultimately held together with string, bubblegum and hopeful wishes, making for a fundamentally absurd and byzantine base-system.
In contrast to mainstream gaming moving on from the number-crunching minutia of the 70s and 80s to a more streamlined, speedier to play and easier to grasp gaming style, a move that even other retroclones tend to adhere to for all their high lethality and low magic, Hackmaster embraces shameless complexity and overly exaggerated rules.
Basically, take first edition AD&D, with its weird class balance, gender issues (e.g. the infamous "strength cap" for female characters), huge number of charts, and idiosyncratic rules, and add a "building points" system, merits/flaws, a huge critical hit table with thousands of potential results, and a ridiculous variety of monsters. It deliberately eschews streamlining and handwaving; you roll for everything, you keep track of everything, and cutting corners is not allowed. It's a bit more coherent than first edition AD&D ever was, but it jumps on any chance it has to add more charts and tables.
The result is a fully playable if murderously complex fantasy tabletop RPG with a healthy dose of in-jokes and meta-humor from the "Knights" comic strip. It reads exactly like the game from the strip, complete with bizarre rules, typographical errors, and lengthy digression-filled rants from Gary Jackson that read like something between a highly defensive, neurotic man speaking out on behalf of his work and the Unabomber's manifesto.
The first version of Hackmaster the game was released in a joking way as "HackMaster 4th Edition" in 2001.
When Kenzer lost the AD&D license in 2007, they rejiggered Hackmaster into a full-fledged and legal retroclone, focusing exclusively on self-designed mechanics so they wouldn't need to pay WotC any more money. This "revamped" version of Hackmaster was released in 2009 as "HackMaster Basic".
In 2011, a 2nd edition of the HackMaster Basic version of the game was released as "HackMaster 5th Edition". Whilst still a comedic game, it had lost much of its previous "grognard mocking parody" ideals, succumbing to market realities and smoothing out its gaming crunch to make a still-complex (for example, combat happens in real time and is based on rolling attack rolls vs. defense rolls) but more easily played game.
Hackmaster 4th edition and Basic takes place in the setting of "Garweeze Wurld", as was the case in the Knights of the Dinner Table strips from which it originated. This is a Sword & Sorcery style setting (with a heavy dash of parody) set on a giant continent, about eight thousand miles across, circling much of the northern hemisphere of the planet Aldrazar. The continent is mapped based on forty-eight "sectors", each one thousand miles across, and reaching from the tropical band of Aldrazar to its Arctic Circle. The setting is fleshed out in the splatbooks "The Garweeze Wurld Atlas" and "The Garweeze Wurld Gazeteer".
In 5th edition, HackMaster switched over to using Kenzer's Kingdoms of Kalamar setting instead.