I've never seen a tool specifically for editing memory card files, but I haven't exactly looked. A hex editor would do the job, but that's like using dynamite to open a stuck jar.
If you can't find any actual gameshark/whatever codes for the game you want, you can go the leet way and use a tool such as
Cheat Engine. It's meant for PC games, but pcsx2 (effectively) is one so it still works.
Google for tutorials, but the basic idea is since all the information about the game state is stored in RAM while playing, you can go in and change pretty much whatever you want with the right tools. Cheat Engine is such a tool.
Start up a game, find the location where max {health | money | ammo} is stored, change it, and save normally. Once you get some practice in it's pretty easy.