You've heard of King of Fighters, You've heard of Marvel vs StreetFighter, You've heard of Marvel vs Capcom, You've Heard of Capcom vs SNK... well this is Everyone vs Everything!!!
You can look for it yourself but it's a bit of a chore to figure out what's going on (those familiar with the first week of learning to use Spring will be no stranger to the feeling), so here's where I'd suggest you start:
Start with "win-mugen_no-limit_2007-10-13_tweaked_hi-res.zip" from the Mungen English Wiki and then download WinMungen Plus
extract "win-mugen_no-limit_2007-10-13_tweaked_hi-res.zip" to a folder and then add the "winmugen.exe" file from "winmugen-hi.rar" to the same folder.
The first package allows you to use the new High Res Characters and Stages (along with low res characters) while the second exe allows you to use low Res stages (which there are an awful lot more of available)... this way you don't miss out on any content thats out there.
I will leave it for now as an exercise for the class to find some awesome characters to share but just to be safe here's some examples:
(DESCRIPTION OF LINK PRECEDES LINK)
Guilty Gear Characters in High Res:
http://drinvincible.hp.infoseek.co.jp/
High Res Japanese Ronald McDonald (Donald):
http://gdgdgdgd.hp.infoseek.co.jp/MUGEN.html
Some Awesome HighRes Stages:
http://yusuke8.blog.shinobi.jp/ (click the link on the left side "Available Stages", then click "2008" to get the download links)
Low Res but has some classics, Peter Griffin, Fred Flintstone, Homer Simpson, Marvel vs Capcom (1+2), Megaman Bosses, Cool Robots:
http://www.infinitymugenteam.com/
Very Limmited Selection and better graphics then gameplay... but they have Bruce Campbell's Ash Williams from the Evil Dead Trilogy soo...:
http://www.mugenation.com/index.php/cat=team/sez=chars
For those of you who feel ambitious and can spare the bandwidth I found some torrents on a quick search that came up with a collection of 200+ characters.
You DO need to manually add characters and stages to your game files so those old text file editing skills you learned from Spring will serve you well and a lot of characters are a lot stronger/weaker then others because they are from different games so you may end up having to tweak a little here and there to ensure balance (or set all the hard characters with order=10 so you don't have to play them in Arcade Mode)... There are "package" downloads available for the faint of heart that include packages of characters and stages.
Oh, and if you DO download, be sure to post fun screenshots. (if you can't see the screenshot I posted PM me and I'll get it mirrored off site)