ROM's for MAME - the MAMEdavs have a FAQ on the legality of them.

[Crazy Cooter]

http://mamedev.com/romsfaq.html

Part of the current guide:
How do I legally obtain ROMs or disk images to run on MAME?
You have several options:

1) You can obtain a license to them by purchasing one via StarROMs or another vendor who has proper authority to do so.
2) You can download one of the very few ROM sets that have been released for free to the public for non-commerical use.
3) You can purchase an actual arcade PCB, read the ROMs or disks yourself, and let MAME use that data.

#1 = www.starroms.com
#2 = Anyone? - don't provide a link unless you know for a fact they're legit.
#3 = www.ebay.com, etc...

--- edit ---
I wanted to add that it's their position.  It was not written by lawyers.

[SirPeale]

#2 = Anyone? - don't provide a link unless you know for a fact they're legit.

There are three of them.  You can download them off the Mame site, IIRC.  Let's see...

Ah, yes:

http://www.mame.net/downmisc.html

[Q*Bert_OP]

and you can -------- roms -- - ---- (censored)