Grey shards were the name given to player-run servers running re-implementations of Ultima Online, to which the official clients could be hacked to connect to. Some of these got extremely successful, and I'm fairly sure many of the developers now work in the game industry. Some were simply opportunities to remove changes they didn't like (like removing the no-PvP lands). Others were completely new worlds. And some had dedicated teams who ran plot and tried to make the worlds seem alive.
Of course, over its years, Ultima Online did go through a number of these changes naturally in one form or another. PvP only servers were launched, and at various times they recruited players to run events and story arcs.
But what kept Ultima Online going wasn't just the old guard PvPers and the addicts to the original 2D low-res client. There had always been a very strong player community on some shards, with NPC towns used as a basis for player plot and story telling.
he tools UO offered were fairly unique as far as this went. You could build your own house, and eventually not only decorate it but design the layout from scratch all by yourself. The player NPC vendor system that allowed people to run shops was clunky, but still very impressive in terms of how creative people got and how many players simply role-played tradespeople and didn't really do the whole PvP/PvM thing. You could place objects on the floor, some of which would even block movement - emphasis added for anyone who hasn't tried any of the popular MMOs to make clear that this still is a rare feature. The crafting system again was very in depth, too.
And then there's fishing. A skill that tbh was pretty boring to use, but combine that with trade and you could enjoy yourself out on the oceans in your boat. Treasure hunting was another path you could dedicate your characters to, which included figuring out where in the world a map was showing, and needed skills in lockpicking and dealing with monsters.
The skill system was again something we don't see a lot of. Mostly this is because it is fairly complex and dull to level up at the same time. Any character could train in any skill, if they had the room to grow or were willing to give up another skill. You could really twink out with complementary fighting skills, be a tradesperson who gathered their own resources, or you could build a completely and utterly useless character, grandmaster at everything that by themselves were useless, and together only had value for roleplay purposes.
And we did just that! Ultima Online had a skill called "Begging", used to get small quantities of money off NPCs. The higher the skill, the slightly more you'd get. Some of us would roleplay beggars, avoiding all skills that we roleplayed as being "evil". We would come up with character backstories, and try to earn money off other players by any lawful means that wasn't using an 'evil' skill or against your personal ethics.
Speaking of that, the things the rules used to let you do against other players, and the fun you could have in the PvP areas... Originally, when you died your kit stayed on the corpse, and in PvP areas anyone could loot it ;)
And if it were not for Ultima Online, we wouldn't have the games we now have. Not only was it the first proper MMORPG (or second if you count Meridian 59), but it also inspired many other games. Jagex, one of the UK's most successful MMO developers was founded by a team recreating UO in java - the game that became Runescape! That game inspired another open-world java game, Wurm Online, which Notch worked on. Notch, of course, later developed Minecraft, and you might be able to see the similarities ;)
Finally, If you're into game design theory, one of the original development team of UO is now a key player in that area of academia, having published A Theory Of Fun - http://www.raphkoster.com/.
[1] Old UO player? Europa? Stratics bod from 2002-2007? get in touch! :D
Edit: And how could I have forgotten to mention the amazing bit of work that was Nick Yee's Daedalus project! http://www.nickyee.com/daedalus/