Xuri's UOX3 Guide offers step-by-step guidance to setting up, running and administering an UOX3 shard. It contains a myriad of tips, tricks and advice for shard admins and GMs, as well as lists of various data such as IDs, light-settings, etc. Recommended read for both novices and veterans.
Ultima Online - Latest Official Client
You can find the latest client on the official UO page.
Ultima Online - Mondain's Legacy client
Grab a copy of Ultima Online: Mondain's Legacy from CNET
Ultima Online Clients at Archive.org
Discover images/installers for other UO client versions at archive.org
ClassicUO is an open source UO client that improves on the regular UO client in almost every conceivable way; resizable gameplay window, zoom control, smooth movement and high FPS, world map, and much much more. Highly recommended. Note that it still relies on the data files of the original UO client.
Razor CE (Community Edition) is the open-source continuation of the previously abandoned Razor project, and is an "assistant" program (similar to UOAssist of old) for player-run Ultima Online shards. Aside from a whole host of quality of life improvements to the client, it also includes the ability to bypass encryption in the regular UO client, and used to be a requirement to be able to connect to UO shards before the introduction of open source UO clients that partially removed this need.
Razor Enhanced is an alternate variant of Razor that comes with an advanced scripting engine and many other improvements and tweaks.
The spiritual heir and replacement to InsideUO, UOFiddler is based on Ultima SDK, and is a tool to view and alter almost every 2D client file. Coded in C# with .net 4.0
GumpStudio is a tool originally written by Bradley Uffner which lets you easily create good-looking custom gumps through a graphical user-interface, and then export those to scripts of various formats (RunUO, POL, Sphere, Wolfpack, etc) based on the available plugins.
NOTE: To be able to export your custom gumps to UOX3's JavaScript format, download the UOX3 GumpStudio Plugin (outdated, might not work properly with latest GumpStudio version) and unzip it to your GumpStudio\Plugins folder. Then enable the plug-in through the Plug-Ins Manager and export your gumps to UOX3's format using the 'Misc' menu.
Quoted from the site: "WorldMaker is a suite of tools that allow a user to create and edit totally custom maps and structures, all from the perspective of how it will appear in the game. The tools are designed to interact with each other, offering the user a seamless experience. less time is spent on fighting the limitations of the tool, and more on the pressing job of designing."
Archive of useful tools for UO, including CentrED (a client/server-based map-editor whose homepage no longer exists)
Shard Portal is a UO Toplist where shard admins can advertise for their UO shards and attract new players to their shards.
UOGateway is a UO Toplist where shard admins can advertise for their UO shards and attract new players to their shards. The name of the toplist and the domain used are a nod back to the UOG client of old that was a combined shard list and client patcher/launcher, but has no other relation to said software than that.
Another nice (German) website focusing on providing custom artwork for UO shards. Amongst other things, it has some very nice animated item artwork (including a moving water-mill).
German website with free custom artwork for UO shards. New cloaks, new hairstyles, new wall styles, new armor types - and more!
Yet another German website with free custom artwork for UO shards. New chests, pillars, equipment, armor, decorative items - and more!
Ultima Online Graphics by Rubra
A site with free (mainly) paperdoll/gump graphics for new equipment, freely usable by any UO shard.
An incredible resource for those looking to designing and decorating houses in UO.
Ryandor.com is the homepage of Ryandor, a true artist when it comes to map-making for UO. His forums are crawling with map-makers from all corners of the world, and if you have a question about map-editing that you cannot get an answer for on these forums - there is no answer ;)
(Legacy link, out of date) The OpenUO project at Sourceforge is an opensource community for the development of Ultima Online emulators, primarily focusing on UOX3.
(Legacy link, out of date) The UOX3 project at Sourceforge.net contains both recent UOX3 releases as well as an archive of older UOX3 versions, and has been a home for the UOX3 code since 2002.
Lonewolf is an UO server-emulator with roots from UOX3 of old, through Wolfpack v12.6.
Heptazane's Stump is the new home of the guy formerly known as Alazane, creator of InsideUO, UO Screensaver, UO Hue Editor and Character Viewer. All useful utilities, but InsideUO in particular was a godsend to every GM on free shards when it appeared. Might not work properly with most recent client versions.