Arcade gaming has always been the very pinnacle of gaming technology of its time, indeed video games initially grew to popularity through the arcades. When people started to own home computers and consoles that they could play games on, the arcade games always had to be a step ahead in technology, graphics, sound and action in order to keep the customers putting the coins in the slot. It is for this reason that arcade games are often so playable and addictive. As home computers became more powerful, the games on them could imitate the arcades pretty well. Today however, the very same arcade games can now be played on most home computers under emulation. Most things are emulated nearly perfectly, except for that insert coin slot. The gameplay is second to none, since the games were designed to keep you coming back for more, and the only games emulated are the ones that the programmers remember as classics - thus you are almost guaranteed quality.
RiscCPS - Capcom Play System emulator written especially for RISC OS.
M.A.M.E. - the colossal arcade game preservation project using emulation to keep information alive.
Retrocade - the high quality, user-friendly emulator, brimming with features and emulating the classics. Unfortunately due to other commitments it looks unlikely that Gareth Long will ever complete this conversion.
There are also emulators for the games Pleiads and Phoenix, written in Java.