A 3rd party software application.
How do you want to host it? Can you install an agent on each client machine? If so – a complete IT software/hardware support application like SysAid Free Help Desk Software would automatically fetch license information for you from each machine. I found SysAid easy to navigate, but the client agent model doesn’t fit our network structure.
Keyfiler is a free web app to file all your license info, but I haven’t personally used it.
I hope these two applications give a good start to finding the right solution.