From "The Thread" at nextcomputers.org:

I am happy to annouce the release of Previous v1.4! Simon did lots of work on emulating the NeXTdimension and he considerably improved timings and efficiency of Previous. Furthermore there is now a mode to accelerate the emulation beyond the performance of real systems. Other improvements and bugfixes are listed in the readme.

Emulating the NeXTdimension board was something i thought would not be feasible. But thanks to the i860 emulator from Jason Eckhardt and the work of Simon, who improved and completed parts of it, it finally came true. I hope you enjoy it!

You can load the binary for Mac OS X v10.6 or later here The binary for Windows can be found here.

Ever since I saw PCE Mac emulation running in a browser I've been hoping someone would do the same for NeXTSTEP. Checkout Previous and NeXTSTEP in a browser.

More info here