Accessing memory of other programs

I used to have this nifty program that i could use when i was playing a game, that i could specify a number and a program, and it would locate that number in memory. Once it did that, i could change that number to whatever I wanted, and it would change in the game. I have recently been researching and programming lots of different things that can be done with memory, and this came up. I would like to know if this is possible in C, and if possible, where I should start looking to find out how. I would also like to know how to look up processes, a.k.a. get a list of them, and their memory addresses.

Thanks!

Processes are compiler/OS specific. Lookup your local documentation.

Here you can find more information. Especially read the section about inter-process-communication. Note that this information may not apply to your environment, as Hammer already said, it is platform and perhaps also compiler specific.
http://www.cs.cf.ac.uk/Dave/C/

With Windows, look up ReadProcessMemory( ) and WriteProcessMemory( ).

for linux you might want to look at shmget and those families of functions.