Ever since Emuparadise took down all of their ROMs, it's been impossible to find ROMs for more obscure systems. This webpage is a list of sites and links to find ROMs, ISOs, and other "pirate" software.
vimm.net - A more "grassroots" site that doesn't feel like some randos trying to get a quick buck on ad revenue. Really slow download speeds, and not the widest variety of systems, though.
Available systems:
romsmania.cc - Pretty decent site, much better selection & download speed than vimm.net.