The major categories are: Federal funds; Repurchase agreements; Certificates of deposit; Commercial paper; Bankers acceptances; Money market funds; Money market futures and options.
Also; take a look at the following:
Fundamentals of the Bond Market-- (call number332.63230973 F127f); page 5; under "Money Market."
Instruments of the Money Market-- (call number 332.408 M748i)