Achievements support
Some time after Super Smash Bros Melee's "challenges" and the Tales Of series' "titles", Microsoft and Sony decided that they wanted to apply the idea to every single title from their library (of course, wiser heads prevailed afterwards and the option to opt-out was given to devs).
Enter the "Achievements" or "Trophies" system on the PS3, X360 and eventually Steam, among other systems. These are tied to the system, though many games (Xenoblade, Kirby Mass Attack, Megaman 9...) had an implementation of the idea independent of the system OS. You fullfill some arbitrary condition in-game and the console displays a pop-up claiming you unlocked that achievement, adding it to a list with some points so that you can brag about it online, or more rarely have some reward.
These achievements range from ridiculously simple and mundane (finish boss 1, finish boss 2, finish game) to intelligent ones encouraging you to play in new ways and milk the game dry (finding rare enemies and doing challenging tasks), to grindy tedious ones (play one million times, kill one billion enemy, beat the trillion-HP boss with an under-powered character one HP at a time).
Some people want this sort of stuff in their retro games. So modified emulators were indeed produced for that specific goal --
Retroachievements.org
This is the download page for customized emulators made by folks at Retroachievements.org based on GPL-licensed emulators with achievement support included. They will appear as overlays during gameplay. The source code is available here.
Shame however that the achievements proper that can be got from the site aren't that interesting (supposedly due to limitations with the emulator's achievement system), but you can develop ones yourself.
- RANes - NES emulator
- RASnes9x - SNES emulator (based on Snes9X)
- RAVisualboyAdvance - Gameboy/GBA emulator (based on VBA-M)
- RAGens - Mega Drive/Genesis emulator (based on Gens)
- RAPCEngine - PC Engine emulator
Common Issues:
- RANes: If you have a black screen at startup, change DirectDraw to Hardware Acceleration for Full Screen.
- RASnes9X and RAVisualBoyAdvance: The achievement overlay only currently supports DirectDraw output.
Steam PS1 Re-releases
One recent re-release of N20 for the PS1 on Steam was found out to be an emulated PS1 ISO bundled with a modified build of PCSX-R (with HLE Bios) to support Steam Achievements.
Per emulator license, the developer released the source for everything but the ISO scrambling part (which was figured out anyways). The achievement system is commented, and relies on monitoring PSX addresses. Tweaked, it could be the basis for a new build of PCSX-R with achievement support. The source can be found here.
