Exult is the masterpiece that inspired all the other SDL-based engines. After having started off as a humble viewer of the Ultima 7 game world, it quickly evolved into a complete engine for Ultima 7, along with many improvements. Development on Exult is complete, and development is now concentrating on improving minor things with Exult itself, developing Exult Studio and developing Pentagram for Ultima 8.
Pentagram started off as an Ultima 8 Map and Shapes Viewer made by some members of the Exult team. This is evolving into an engine in itself. While the game is not yet fully playable with Pentagram, it is fast becoming an Exult for Ultima 8.
Seven Towers is the website of Marzo Sette Torres Junior, a member of the Exult team. It contains some Exult mods and Usecode C documentation.
The Ultimatrix Homepage is home to many brilliant utilities such as the Ultimatrix viewer and the Ultima Launcher.
Gargish Dragon, as the name suggests, knows a lot about the culture of the Gargoyles and has written useful information about their language, including a Gargish dictionary. His dictionary received a link from the official Ultima Web Archive, even though that link is now old and doesn't work.
Nuvie (meaning "New Ultima VI Engine") is developing into a nice engine for Ultima 6. While there are no official releases yet, test releases (aka snapshots) are released regularly. While not everything has been implemented, Nuvie is not only becoming an engine to run the original game with, but also has lots of impressive improvements that the original game lacks.