I don't think your question sounds all that nefarious, so I'll help you in your quest for knowledge. If you should use this to steal others accounts, I hope you get what's coming to you! Otherwise I hope I can help.
Steam stores everything about the users that use a machine in a file called ClientRegistry.blob. This file is located at:
C:Program FilesValveSteamClientRegistry.blob
The password (when you hit "Remember my password") is stored in ClientRegistry.blob. The password is MD5 hashed by steam. But it is possible to find the saved password if you make a call to the steam.dll file to decrypt it, this works because of a serious implementation flaw by Valve.
Luigi Auriemma has already written a hack to decrypt the stored password. Now a history lesson in stolen code. After he wrote this code, someone stole it and wrote a version that decrypted the password and then sent it to them so they could steal every steam account for everyone that used it. Luigi's version IS NOT A TROJAN, but antivirus software will flag it as such because it is a huge chuck of a trojan (again, they stole his code for part of it). You'll have to disable your antivirus for the program to be able to run if it flags it.
This is the decrypter he wrote:
http://aluigi.altervista.org/pwdrec/steampwd.zip
This is his other hacks for decrypting passwords:
http://aluigi.altervista.org/pwdrec.htm