Leitax, but it is not exactly an adapter but a replacement bayonet. Leitax is here: http://leitax.com
Regarding lenses, your 28 mm on the D50 corresponds to about 20 mm on full frame 135 (based on image diagonal). So I guess 18 mm is closest.
I would also consider Zeiss lenses and some of the Nikon lenses. Sigma 35/1.4, Sigma Macros are said to be awesome.
Zeiss has a new generation of lenses built for full frame 135, Distagon 25/2, Distagon 55/1.4 and Apo Sonnar 135/2.
I'm pretty sure many of Nikon's lenses are perfectly good for the job.
I am not sure that Leica lenses are better than say Zeiss, Nikon or Sigma. They may be better in some cases or some aspects.
Most lenses are quite OK when stopped down, so much of the effort is going into designing lenses that perform well at large apertures.
Photozone has a lot of lens test. They are good tests as long as we agree with their testing methods. They "work around" field curvature and it may be OK or it may not.
I have not seen a lot of comparisons between Leica and Nikon.
Marc McCalmont (who uses Leica lenses on Nikon D800E) is very helpful, you can consider PM-ing him with your questions.
OK, so let's say I go with the D800E as I'm obsessed with sharp landscape details.
I noted on the link above to the Phase/D800 comparison that he was using a Leica 50Cron lens on the D800.
Search shows there are several Adapters for the Leica to Nikon lenses.
Any favorite brands?
Has anyone compared the Leica lenses with an adapter to the Nikkor ED or like lenses?
In short, what I'm hoping to define is what wide angle (I live with the 28 Hassie as my main lens) and shot tele lenses, regardless of price, would be considered the optimum to mate with a D800E shooting landscapes.