MCM:
http://www.mameworld.net/mcm/I don't really like mcm all that much, since it does miss a lot of games in the categories, but it IS a good starting step.
To further pare down your ROMs, check out ROMCenter ... romcenter.com with mame dat file from logiqx.com. NOTE, when running RomCenter the first time, you MUST choose a dat file from the combo box up top right (it's not obvious...I spent a good 30 mins trying to figure out how to run the prog). After that, things are really great. RomCenter is very good at getting rid of clones as well.
To further pare things down, some more search tricks you can do in windows, in your ROM directory...
do a: *mahj*.zip, search, move the results into another folder. Also, if you don't like super old games, you can sort your ROM directory by filesize, then move everything <8k to some old folder.