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.
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.
Among Paul Robson's SDL Games is an SDL remake of Akalabeth.
Labyrinth of Worlds is a remake of Ultima Underworld 2 with such things as 3D models and a different GUI. The feel of the game is being kept as close to the original as possible, but this remake is not going to be as faithful as Exult, especially as far as graphics go.