See also How to Play for more details about installing each shard.
For a list of historical shards, see the Guild of Archivists’ wiki page.

A shard is an array of one or several game servers that support a instance of Uru Live. In other words, shards could be considered Uru Live private servers, with the caveat that they are (usually) allowed and encouraged by Cyan, which released the client (but not the server) code as open source.

Myst Online: Uru Live (again)

Maintainer: Chogon

Cyan’s official Uru Live shard, often referred to as MOULa.

Minkata Prime and Alpha

Maintainer: Rarified

A pair of Cyan-like shards maintained by OpenUru.org, meant for adapting/testing patches and fan Ages to be added in MO:ULa.

Gehn

Maintainer: Hoikas

The Guild of Writers’ bleeding edge (but not testing) shard, with various improvements (in the process of being ported to MOULa) like accents in chat, improved audio chat, better visuals. It has an old version of the Fehnir House fan Age, not yet on MOULa.

Deep Island

Maintainer: Diafero

A shard that requires the offline (Uru: CC) version of the game as client. It has the largest collection of fan Ages.

The Open Cave (TOC-MOUL)

Maintainer: Filtik

A shard that includes a crazy, fun modifications of Ae’gura named the City of Dimensions, a boat trip around the Lake, and a pet. It has an old version of the Fehnir House fan Age, not yet on MOULa.

Keep in mind that the shard is not actively maintained. A specific set of game files is required for this shard to work correctly; you can get it from PodHopper’s page or Korov’ev archive.

The Open Cave (TOC-PotS)

Maintainer: Filtik

Distinct from the one above (you will also need a separate login), similar in many ways to Deep island. Fan Ages are not installed on the fly, though, they must be installed with Drizzle before launching.

Keep in mind that the shard is not actively maintained.

Destiny

Maintainer: Mystler

A shard meant only for alpha-testing fan Ages, thus not intended for regular gameplay; some areas (like Ae’gura) are in their default state, with barriers on.