To clarify things, I don't care if it runs in OpenGL or not (although I don't think a D3D port is possible without completely changing everything). I just care about better visuals. :-)

Thanks for all suggestions! Helpful links will get rep. ;-)

In case you want some classic gameplay on a modern hardware, try zdoom:
http://zdoom.org/Download

While searching for some of the ports here I came across this page:

http://doom.wikia.com/wiki/Source_port

It seemed like a good summary list(scroll down).

Also, this one buts its more a repository and age of ports can be in question:

http://www.doomworld.com/classicdoom/ports/

and of course the official wikipedia page on them:

http://en.wikipedia.org/wiki/Doom_source_port




Note a simple Google Search for GLHexen gave me this:

http://www.gamefront.com/files/9986/74ad6e4327d39444af2c6246ce5310613497b07def50e052d5ead58c6958a6332035b0b1d21e94e2

Unfortunately the game file looks quite old there but at least the site is trustworthy(they were filefront).

Tried Skulltag. The graphics will win no beauty prize, but the games are playable even for modern eyes again. They look like 1998 now instead of like 1994. ;-)