Comment Start with disaster scenarios (Score 5, Informative) 528
For me, visio's are great and everything, passwords too, but really the most valuable thing you can do is document single points of failure, outdated software/hardware, etc., license keys/expiration dates, cert expiration dates, personal support contacts you have and all vendor relationship details as well are essential. Do you use change control? If you do, go back and comment your changes, if not, do the best you can at explaining why things are the way they are. Get some open source software that is good at indexing data and create a searchable knowledge base from the information above. Don't concentrate on docs that can be found on the web at first because any admin worth their salt will know where to look for how to's, etc. Focus on the why's, the where's and disaster recovery.
My two cents...