Unreal Tournament had a decent mod scene too though, and it was only with HL2's Source Engine (and I suppose Valve's active support of modding) that I saw people leave that scene.

I only worked on mods for UT and the original Jedi Knight, and I've only heard second-hand that UT and Quake (3? 2?) were easier to mod than HL1, so I might be wrong.

And compared to Jedi Knight and its only editor(s) being user-made, all these games were easy to mod!

Well, you barely hear about the UT mode scene. Now think of three Quake mods, quick:

Team Fortress, Quake Chess, AirQuake, ThreeWaveCTF, Xonotic (kinda, sort-of)

Think of three Half-Life mods, as fast as you can:

TFC, Counter-Strike, Natural Selection, Sven Co-op, Science & Industry

Think of three Quake 3 mods, as quickly as possible:

OSP, CPMA, ThreeWave (again), OpenArena

Think of three Doom mods/mappacks, fast:

Memento Mori, Brutal Doom, Batman Doom, Russian Overkill, The Sky May Be, Tech Gone Bad, and so many more

Think of three Minecraft mods, as fast as you can:

Thaumcraft, IC2, Buildcraft, Forge, FMP, Thermal Expansion, Better Than Wolves, RedPower, NEI, Ars Magica, Applied Energistics, Aether Mod, Tinker's Construct. You want more?

Think of three UT mods:

Ummm... Errrr... Ummm... If you count mappacks, somebody might have made a version of Aerowalk for it?

Now, that's just me, but I doubt others would have dissimilar responses.