Is there any free tools out there that can help to better document at datacenter, for information like servers in a rack, the group port it is connected to and etc?
Check out RackTables: http://racktables.org/
We're using MediaWiki  in our company, works really well for internal documentation. Powers wikipedia as well.
You can use nVentory - Ruby on Rails Datacenter Inventory Manager.
You can also look at http://projects.autonomy.net.au/hotwire. It's a network and DC-aware inventory tool for servers, racks and peripherals.
Rackmonkey does the basic rack-part ok. (unless you have blades)
If you want to document more you could use GLPI: http://www.glpi-project.org/?lang=en which has a rack plugin. You can document servers, switches and devices without the plugin.