Recently came across these two very comprehensive lists of open-source game engines for proprietary games edit: and various clones and inspired projects:
I feel the latter has an a bit misleading title, as none of those are game clones (e.g. similar remakes) and most of those are based on the liberated original source-code also… but well I guess people get what the list is supposed to be about (Edit: see comments).
However talking about an actual engine reimplementation project: OpenMW has recently released a new update (0.23.0) with some impressive additions (as usual) and version 0.24.0 is already tested as a RC right now.
Feel free to comment about this release on our forums!