This is the most important section. MAME itself is legal. However, downloading a "ROMs pack" containing copyrighted games (Pac-Man, Street Fighter, Metal Slug, etc.) is copyright infringement.
MAME itself is legal emulation software. The developers argue that ROMs should only be used for: mame32 all roms pack
If you want the experience of a full MAME collection without the headaches of ancient MAME32 packs, here is the modern, recommended approach: This is the most important section
Split sets are organized with parent ROMs separate from clones. This saves space if you only want parent games. here is the modern
To recreate the arcade feel, download: