Thanks to the generosity of kerio, DynaHack can now be played online on the nethack.xd.cm public NetHack variant server. Watch for DynaHack deaths, ascensions and discussion in the #nethack.xd.cm IRC channel on freenode.net! Information about nethack.xd.cm across this website has been updated accordingly.
If you want to play DynaHack with numpad controls and use PuTTY, you will need to disable "application keypad mode" in PuTTY's configuration; see the DynaHack FAQ for more details.
2015-08-17 update: DynaHack should now work as-is with numpad and PuTTY. Also, check out the new nethack.xd.cm web terminal!