One of these was the 'Giant Killer' kit - a variety of ranger who made it his business to hunt and slay giants, and got certain advantages when fighting them. I used to want to run a campaign in which the PCs were all Giant Killers roaming the wilderness, protecting the weak and timid from gargantuan monsters.
A few years ago I wrote a post about the giant slaying campaign. I described it as:
[R]evolving around infrequent encounters with large, powerful, and possibly unique enemies right from the beginning. In this paradigm, monsters would start off at the level of trolls and progress from there; they would be mighty and perhaps near-legendary beings who only true heroes could possibly hope to defeat in a fight: Fafnir, Gog and Magog, Grendel, Glaurung, the Green Knight. Giant Slaying adventures would involve much in the way of tracking, exploring and inconclusive battle, and comparatively little in the way of slaughter.
I used the old Playstation game The Shadow of the Colossus as an example of how this would play out. There would be no goblins or kobolds or anything of that nature. Only big, ferocious creatures that you would have no hope of defeating without team-work, strategising, and forward planning.
It didn't occur to me at the time that the Giant Killer kit existed - I'd forgotten all about it. It was only leafing through the Complete Ranger tonight that reminded me of it. It's tailor-made for that sort of campaign. I particularly like how a party composed entirely of Giant Killers would be specialized to the extreme - they would be excellent at working together to hunt down and destroy giants, but correspondingly vulnerable against magic-users and human enemies. This would make an interesting strategic problem for the players: perversely, they might be better against big enemies than those their own size.
The only problem with the Giant Killer campaign is that D&D combat is too abstract to do it justice. You don't particularly need to be creative when, despite a giant's size and unique challenges, ultimately all you are doing is just rolling a d20 against AC like you do for any monster. I wonder if there are any other systems which are designed to handle small human versus big monster combat?