I have various libraries. Each library has its own subject, such as mathematics, hashing, security, bit manipulation. As I program in different languages and not all code is portable, I just tend to keep those libraries near the developing environment, as they clutter up quickly.
Each library consists of one or more well commented and well named functions. They range from Counting the number of set bits in a 32bit int, to deriving some unique machine id from the network interfaces and corresponding MAC addresses.
Projects (this is handy in .NET environments, you can load them everywhere) and compiled libraries (.dll).