I have been searching out emulators for ZX spectrum / Arcade / Amiga 500 mainly and found some good sites.
First of all MAME works out of install with many arcade roms for all old classic cabinet arcade machines:
MAME (arcade) can be downloaded here:
Games for install "Roms" (you place them in the "Roms" folder and let the software validate them)
NOTE: All other emulators (MAME runs out of install no problem) require bios or kickstart (amiga) files for them to boot, that is a legal issue and there are many sites that you can download kickstart amiga boots & zx spectrum bios from that are payment, clearly that is in the realms of legal etc so its up to you to google as you wish, Im not endorsing non payment download here, everything I have I have payed for. If moderators feel this is dodgy ground then feel free to remove, although all sites linked only supply the emulator and games, bios & kickstart files are not supplied.
Amiga (winaeu) can be found here: (click alphabet)
Having some fun playing Way Of The exploding Fist, Lesuire Suit Larry, Outrun, and simply laughing at some of the games I loved in my school days.