There have been a number of warlock classes, but unlike the wizard, fighter, cleric or even thief, everyone has had their own take on what a warlock should be.
I have talked about the warlock as a class, distinct from the witch, in the past.
I think the first ever warlock class I ever saw was the "Warlocks: A New Magic-User Sub Class" by Anthony Barnstone in The Dungeoneer #16. It had some great spells, "Pentacle of Fire", "Aura of the Occult", "Curse of the Bloody Revenge" to name a few. This was certainly meant to be an evil character class to play, not just as an NPC. Interestingly enough this the same issue that featured the mystic class. I have to admit it was one of the things that made me like the Dungeoneer magazine. It didn't treat it's audience like little kids.
To my knowledge, there has never been a warlock class in the pages of Dragon magazine. I know there was not one in the pages of White Dwarf.
The Arcanum and Bard Games had a witch/warlock class, making them the same thing. I am not a fan of that really.
In my mind the witch and the warlock began as the same class, but the warlocks broke off from the witches sometime in the ancient past. Either warlocks wanted to become more like wizards and mages OR they were responsible for the first wizards.
Astonishing Swordsmen & Sorcerers of Hyperborea has a great Warlock class. It is a sub-class of the fighter but calls upon dark powers to give them some magical power and spells. Mor to the point I like how the warlock and the witch are very different sorts of classes.
D&D4 Essentials Hexblade. In general I liked the D&D4 Warlock. They were a class that wanted quick access to power and none of the work that Wizards had to do. That was a fine role-playing excuse, but not something that played out in the rules. Warlocks gained powers just like the Wizards did and had no more or no less requirements.
There is a Warlock I created in Eldritch Witchery. It is a type of Wizard really. I liken it to "Wizard Grad School" to be honest. They use the same spells as the witch and gain a few extra powers.
The Warlocks in Fantastic Heroes & Witchery are another sort. It is a chaos aligned wizard and has a lot of the same features really. It uses the same xp per level tables, same HD and same spell progressions. The FHW Warlock does gain some power, similar in many ways to my own witch, but at a cost. On the surface this doesn't make it much different than a wizard, with a different selection of spells. What makes this class, and really this book, different are the selection of spells (the book has 666) and the additional rules for acquiring magic and casting spells. Adding this material makes the Warlock a much more interesting character.
The Pact-Bound in Magical Theorems & Dark Pacts is another warlock-like class. Again the idea here is a class that takes a quick path to power for a price, usually to an other-worldly power.
There is a similar one in the pages of the ACKS Player's Companion. Again the nice thing with this book is that the witch and warlock are separated.
In the 3e era we have a couple of "warlocks". There is a warlock in the Complete Arcane and the witch in Pathfinder, which always felt more like a warlock to me. Just staying focused on 3e we have a warlock class from WotC and a witch class for Pathfinder. For 4e there were also very different witch and warlock classes. 5e only has a warlock.
In the case of the official D&D warlock, he is less of a spell caster and more a raw magical power wielder. His pacts give him this power.
The question becomes one of whether the warlock should have spells or just weid raw magical power and thus have "blasts". I am torn myself. I like the warlock to have access to spells to be honest, the idea is these guys have sold their souls for power, but the "blasty" warlock really isn't all that powerful compared to a "spelly" warlock or wizard.
A good example of what I call a "blasty warlock" is Jeremy Reaban's The OSR Warlock. Like his Witch Hunter book this book has a number of nice features in addition to the class. The class does not cast spells, it does have lot of special powers. This is by design and owning to the stated OGC and pulp sources. The warlock here does get some spell like abilities in place of powers. It actually works rather nicely What I think makes this book special is the level advancement tables for "First Edition", "Original Edition", "Basic/Expert" and "Cyclopedic Edition". Plus the author has a section of notes on the class.
I have to admit one of my favorite "warlock" books and one that captures the Pulp Era warlock well is Green Ronin's "Warriors & Warlocks" book. Yes it is for their superhero game Mutants and Masterminds (2.0 version) but it was my goto guide for a proper pulp warlock will AS&SH came out, and it is still a lot of fun.
I am certain I have missed some here. Let me know in the comments below!