I agree OPmanager is good however its limited to 20 devices.
Check out The Dude http://www.mikrotik.com/thedude.php
This application is freeware and works really well for creating topology diagrams and monitoring in the one package.
My other recommendation is for SYDI which is an open source network documentation project.
I have nothing but praise for this tool and the effort that the programmers have put into it.
You can document not only standard servers but Exchange,SQL and LINUX servers!
This one tool is worth its weight in gold as you can run it against a remote system and pull all the info into a word document.
It will not work across a VLAN that is the only issue I have had with it for some reason.
http://sydi.sourceforge.net/
With these two tools you have no excuse for not having network and systems documentation.
For job ticketing and help desk I recommmend Liberum which is also open source and based on ASP, I use it on my network and it has never let me down.It is very customisable and the support forums users are very helpful.
Best thing about all these things is its all FREE.
Cheers