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.
Ultima 2 in Windows is hosted at Ultima: The Reconstruction and can be used to run Ultima 2 under Windows. Unlike the other engines listed here, this does not seem to be cross-platform and focuses only on Windows. The last release dates back to 2000, and the project does not seem to be maintained any more, but that doesn't matter as it is complete after all. Along with the ability to play the old Ultima 2, there are also options you can set to your comfort, such as the screen resolution, the colours and the ability to easily restart the game.
Ultima 3 in Windows, or U3Project as it is also called, is based in Windows (just like Windows-Native Ultima 2 above). Render Dragon is hoping to rewrite the game as a Windows application using the data files of the original game. This rewrite involves other improvements, especially as far as graphics are concerned.
xu4 allows you to play Ultima 4 on modern operating systems, and is cross-platform as it uses SDL. Although the game is going through beta testing, it is supposed to be fully playable from beginning to end.
Only the name is known about this project. Nu5, standing for "Neat Ultima 5", has no website content and appears to still be in planning stages.
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.
Brad Hannah is building an engine around Ultima 5's original game files, and gradually moving in the direction of an isometric 3D perspective.