Documentation is pretty important to an IT organization. Without it, there's no good way to transfer knowledge quickly when it's really needed.
If you are the only network guy, or server admin in your company, if you don't document well, it will be very difficult for you to handoff responsibilities to someone else. This means you can't get sick for very long or take decent sized vacations.
So, just about everyone knows that documentation is important, but what should you document?
If you are THE IT guy in your firm, or are involved with networking or servers, the first thing to create should probably be a network map.
For this, I highly recommend Microsoft Visio as there are plenty of shapes and stencils that are made specifically for it to represent specific makes/models of equipment, although you can pretty much use any drawing program to do it in a pinch.
There are some automated tools, including LANSurveyor (that integrates with Visio) and cheops, that will create a network map for you, but you will still have to do some tweaking of them.
If you need to get an idea of what a network map should look like, go to Rate My Network Diagram
There are plenty of examples of what to do and what NOT to do when building a network map on that site, for all sizes of networks.
Some other things to document are a list of IP addresses and their associated systems for your internal and external networks (I usually do this in Excel) and it would also be a good idea to keep a list of all your passwords, in a very secure location of course.
Startup and shutdown procedures for your servers is oftentimes useful, particularly if they need to be brought up or taken down in a specific order.
Another suggestion is to write up a procedure of your backups. Make sure it's simple and comprehensive enough so that anyone in your organization can follow it as backups should be performed regardless of status of the IT team.
Might also be a good thing to put a lot of this information into the IT section of your Disaster Preparedness/Business Continuity Plans.
At any rate, document as much as you can.
Also please keep in mind that hoarding your knowledge is not a good way to ensure job security. What you know is much less important than what you do with that information. Plus, if you are the only one who knows the systems, you won't be taking much time off.