Just for entertainment with a little bit of possible gambling?
http://forums.bukkit.org/threads/fun-mobarena-v0-87-1-become-a-mob-fighting-gladiator-818.19144/

This one could be useful for underwater cities and traps.
http://forums.bukkit.org/threads/mech-floodgates-1-2-trapdoor-water-win-818.18668/

If the mcMMO is going to be removed, i have a sugestion for replacement.
http://forums.bukkit.org/threads/mech-the-minequest-v0-52-rpg-system-with-customizable-quests-classes-and-npcs-818.8576/

This might be useful if the server will turn mostly PVP. Retractable Bridges
http://forums.bukkit.org/threads/mech-retractablebridge-v1-3-6-easy-and-realistic-retractable-bridges-556-818.10659/
Aand there another plugin which works great with the previous one. Draw Gates (that's how you say it in English?)
http://forums.bukkit.org/threads/mech-portecoulissante-v1-2-1-easy-and-realistic-portcullises-556-818.7346/

Another plugin, miht be useful for sieges >. TNT Cannons
http://forums.bukkit.org/threads/fun-mech-aimcannon-v0-5-aim-with-this-tnt-cannon-684-733-815.13568/

Another RPG Plugin:
http://forums.bukkit.org/threads/fun-mech-pwncraft-v1-06-the-rpg-pvp-plugin-740.16185/

Gambling (Another source of income):
http://forums.bukkit.org/threads/fun-mech-gambling-cards-dice-and-signs-740.15626/

Something for peopel often stuck in world holes after teleporting:
http://forums.bukkit.org/threads/fix-mech-bananachunk-v4-6-stuck-in-a-lag-hole-request-a-chunk-resend-now-with-auto-resend-818.19232/

Some jobs for bored players: Jobs
http://forums.bukkit.org/threads/econ-jobs-v2-0-1-jobs-plugin-for-minecraft-818-iconomy-4-5-boseconomy-permissions.13294/

iConomy. It's quite to play without a shop, huh?
http://forums.bukkit.org/threads/econ-iconomy-5-0-1-eruanna-779.40/

Some other economy plugin:
http://forums.bukkit.org/threads/econ-localshops-v3-1-1-fifth-avenue-economy-based-player-shops-766.17131/

So the people will remember how bad you are at this game: Tombs
http://forums.bukkit.org/threads/fun-tp-tomb-0-5-8-make-people-remember-you-after-your-death-818-permissions-iconomy-4-5.14819/

Useful in case if someone has something forbidden (or is trying to hide something): Item Detector
http://forums.bukkit.org/threads/mech-fun-itemdetector-v1-1-an-item-detecting-gate-818.14416/

I really like the Mob Arena one... It looks likt fun

So we can haz Mob arena? )

Yay its like Christmas except not christmas!!

We should get the Mob Arena Plugin. The arena now isnt all that great