A pure-Java implementation. Possible largely because of BitSavers and David Pitts efforts.
Ever since I first saw technical data on the TMS9900, I have wanted to program in this architecture. Since there exists actual ROM, tape, floppy, and harddisk images for software, it seems possible I might be able to accomplish that goal.
Current Status: Basic model TI990/4 system is working, with ASR-733 cassette load and FD800 floppy disk boot (of TX990 OS). Next steps: TI990/10 with DS10 and booting DX10 OS.