Help Desk Tools

As part of the IT Support & Services that I am trying to build, I came to understand some of the tools necessary to support IT Help Desk functions. And due to budget constraints, I have to rely on Open Source / free tools to do those. Ans since most of the good ones are not free, my choices are limited to the following:

Windows tools:
1. SpiceWorks - This is a freeware monitoring tools with a nice GUI and has a support forum for users to collaborate.
2. Look @ LAN - A very simple scanning tools for IP addresses & other Windows related info; however, some of the Windows firewall configurations might need to be disabled / opened.

Linux tools:
1. Nagios - The best Linux based monitoring tools. There are a few other alternatives such as Hyperic, Pandora, or OpenNMS, just to name a few. However, none can compete with Nagios in terms of support & popularity.
2. SysAid Free Edition. I have experienced with the paid version in my previous company, but it was installed on Windows system. Now it has Linux version, and so I think it might worth to get a second look.

Web-based tools:
1. Trellis Desk - free web-based trouble-ticketing system, easy to configure and support localization (I have translated it to Indonesian) :-)
2. OTRS - Open source Ticket Request System with various features to manage customer telephone calls and e-mails
3. Free Helpdesk Software - A very simple helpdesk solution

Do you have any other tools that is worth to be included in the list, please inform me on the comments below. Thanks.

Updates

I sincerely apologize for not posting anything for the past few months. It has been the ups and downs of my career, and now I am starting my own business providing IT service support. There are lots of information in the internet on the tools to do this, so I am planning to share it here in due time. Hopefully it won't take too long... :-)