Andrey Budko:
He shows some amazing skills with jHeretic; jumping his way to normally impossible times with some cool shortcuts.
For those with patience for the Doomsday interface, these are highly recommended.
Various nightmare demos:  download / Mirror-- includes playback instructions for jHeretic.
Episode3:  download
Episode1:  download

Drew "stx-Vile" DeVore:
hep1sk5 -- full-episode nightmare speedrun - Zdoom2.0.5.2 only -- 9:12
hep1sk5new -- full-episode nightmare speedrun - Zdoom2.0.5.2 only -- 7:58
hep2sk5 -- full-episode nightmare speedrun - Zdoom2.0.5.2 only -- 6:33
hep3sk5 -- full-episode nightmare speedrun - Zdoom2.0.5.0 only -- 12:53
hep3sk5new -- full-episode nightmare speedrun - Zdoom2.0.5.2 only -- 8:01
hep4sk5 -- full-episode nightmare speedrun - Zdoom2.0.5.2 only -- 11:14
hep5sk5 -- full-episode nightmare speedrun - Zdoom2.0.5.2 only -- 12:30

Robert "Ichor" Eckhardt:
cr2437s4 -- Crypts - maxdemo - Zdoom2.0.4.7 - 24:37
cr0852s4 -- Crypts - speedrun - Zdoom2.0.4.7 - 8:52

Jeff Easthope // Branimir Beric:
htic-nm -- BlackPlague -respawn 'nightmare' demoset for Episodes 1, 2, and 3. -- these are heretic.exe 1.3 demos... some desynced for me.

Vincent "Peroxyd" Catalaa // Michael "OgreSlayR" Lastovica:
hkcptrik -- Cooperative speedruns on e1m1 and e1m7 -- these only playback with Heretic v1.3 and Vv's exe modification(included).

Bruno Vergílio:
hes5 -- -nomonsters 100% secret demoset for Episode 5. (E5m8 is skill 1) -- these playback only with Heretic.exe 1.3 and Legacy 1.42.
 

Hexen

Drew "stx-Vile" DeVore:
hubtrk1 -- nomonsters Hub trick cheated (for simplicity) speedrun - Zdoom 2.0.50

Cacodemon Leader:
hexsssk3c -- Entire game - Skill 3 Cleric speedrun - Zdoom 2.0.98 only -- 40:46 -- includes numerous tricks including one huge glide shortcut.

Relevant Links:
DSDA Tools directory -- Source ports and demo tools.
Pdang -- Heretic demos.
Heretic-N -- wHeretic demos. Heretic NoMonsters demos needs the command "-nomonsters" to be played back correctly, otherwise monsters will be present. All valid Nomonsters times are based on the time the command "LMPC -I -gH mydemo.LMP".
Doomworld patches -- Heretic/Hexen patches.