madth3: Even if the game had no achievements. If the format of a save file was known, maybe some achievments could be created?
For example you could tell when the player had reached a certain level and give him an achievement on that.
Note: I'm not for achievements or against them. Just interested in the technical side of things. :-)
The only game I've modded is Baldur's Gate. The save file format is well documented and it stores global variables. It would be extremely easy to create a mod that would set a variable once an achievement is, er, achieved and this could be stored in the save file and read by the client.
The only issue I see, is that using save files for achievement would make it extremely easy for anyone to get all of them simply by editing a save file.
Not that achievements is something I'd want to see in those old games. But I'm not really that much against them either.