^ World of Minecraft stilll exists today, and still gets updates too, despite being for Classic. (at least, I THINK it still gets updates)
I still have a few of my jars from Alpha actually, and a few notable mods on those include:
> Hookshot
> Ultimate Fist
> "Autocart" (Basically, punching a specific side of your minecart made it go in that direction, max-speed.)
> Paint Your Blocks mod (I still think this was better than the coloured wool we got)
> SDK's Gun Mod (which isn't a "reskinned bow an arrow", despite threadular belief - SDK even had a working stationary turret in there, too!)
> And I have another mod which lets you modify and create your own recipes - I believe it's part of Zombe's modpacks.
These all pre-date ModLoader, iirc - as I recall ModLoader originally being created to solve mod conflicts which used the same part of "items.png", as to add a mod, you had to manually edit the mod textures into items.png - and if two mods used the same part of the file, they're getting the same sprite. That, and ModLoader also helped with class conflicts.
For server mods, I would think either RuneCraft or hMod would be the first ones - I can't quite remember which came first, but I know RuneCraft was a mod of the vanilla server long before it got ported to hMod.