Here's a nice little run down of some games that are celebrating Halloween with some nice event items, quests, tasks and other fun activities.
World Of Warcraft
The realms of WoW celebrate most holidays, they've even got an in game calendar that allows you to check when things are.
Previous Halloween events were based around the head less horse man and had players using water buckets to put fires out, as well as some extra dungeons with rare drops and mounts.
This year is the same with some minor things altered, defenetly worth checking out If you're a WoW player.
Team Fortress 2
The guys over at Valve have treated us once more, this year bigger and better than any other year, introducing TF2's first Boss.
The Horseless Headless Horsemann.
Valve once more take a very tongue in cheek angle, providing some nice hats, achievements and a new level based inside the Mann Mansion.
Check it out If you've got TF2, I've spent hours on this event so far.
Announced earlier this month, the Halloween update has finally landed with some nice features, listed below with a tick list courtesy of Notch himself.
The new Hell world is accessible through a portal that must be made of an Obsidian frame 4x3 and then the bottom blocks must be set on fire.
I'm yet to explore the Hell realm myself (The Slip is the official name) because I've been playing with the Biomes that were also released today.
The Slip and all of today's updates are only available in the purchased Single player and Multiplayer Survival modes.
Adventure Quest/Dragon Fable/ Mech Quest
Referred to as Mogloween within this Universe. These are all browser run RPGs, generally quite fun and a nice little time sink and it's free to play (To some extent).
The Halloween event will have you fighting beasts, trick or treating and gathering event based weapons - Not a bad little hour's worth of fun.
Whilst Gaia Online is mainly an anime based forum, it also has a browser run MMORPG built into it; It's nothing to write home about and it really only appeals to those that enjoy the forum side of the site, but it might be worth checking out.
Transformice is a charming little game that features Mice and no transforming oddly enough.
The basic aim of the game is to get the cheese by solving the puzzles within each level, it's a multiplayer game and sometimes you have to rely on the help of team mates.
The Halloween event on Transformice consists of flying on a broomstick to collect cheese, it's a random event that pops up every once in a while whilst you play through normal stages, so go check it out and get addicted!
Gears Of War 2
Currently hosting a double EXP weekend for Halloween, worth checking out If you enjoy the online.
I believe that Dude Huge mentioned that those who've done extremely well in Gears 2 will receive something in Gears 3; I believe he tweeted it earlier this year - But don't quote me directly on this.