Yeah, I spent a lot of time with that title... heh. Anyways, you can find just about anything Siege related right here!
SIEGE by Raven Entertainment (added 3/11/99) - This is the by far the most popular mod for HexenWorld (bearing in mind it was pretty much the only mod...). A team game, it places one side as the defenders and the other as attackers, the goal being to sucessfully besiege a castle. Really fun! Definitely a must have! (5.87 MB)
Be sure to go to the Other Mods page to find some great siege-related mods.
Also, head over to the Siege resource page for all the info you could want on the game, including how to run a server.
Siege Map Packs:
Stables Siege Map Pack Vol. 1 (added 3/11/99) - a collection of maps for siege, includes all below with the sign beside it. (9.18 MB)
Individual Siege Maps:
Besieged 2 by The Partisan (added 3/11/99) (2.65 MB)
Desert Siege by Kain (added 3/11/99) (2.13 MB)
Druid's Keep by Kain (added 3/11/99) (1.91 MB)
Winter Siege by Kor Skarn (added 3/11/99) - basically a version of the original siege with new snow textures... cool. (2.82 MB)
Gorge by Boog (added 3/11/99) (1.06 MB)
Crypts by ShadowLord (added 3/11/99) - a cool map with a nice horror-based theme. (2.86 MB)
Dominion of Dark Fortress by Dark Ravager (added 3/11/99) (3.33 MB)