Spiceworks and OTRS are excellent programs. Both are web based applications.
A basic comparison:
|Free (cost) software||Yes||Yes|
|Open Source (you have the source code)||No||Yes|
|Installation is cross platform||Windows Only||Yes|
|Client access through a browser||Yes||Yes|
|Customizable with your company brand||Possible?||Yes|
|Collects inventory information on your LAN||Yes||No|
|Connects to your database||No||Yes|
|Integrated forums and web support||Yes||Not integrated, but available|
|Monitors your network and Exchange Server for errors and usage||Yes||No|
This breakdown is by no means exhaustive. Spiceworks is a software you'll want to use for gathering information about your network. For free, Spiceworks can't be beat for all it does. In my experience, Spiceworks is slow-ish. It would also help to have Spiceworks on a machine that's dedicated for its use. A Windows Server (vs. Windows Desktop) is a likely candidate for where to run Spiceworks. Windows XP Pro is adequate, though some scanning of large networks will possibly cause errors regarding networking/connecting simultaneously to multiple computers. Spiceworks also has a helpdesk module, which is functional. Some of Spiceworks' helpdesk strengths include the agent shortcuts that allow agents to manipulate tickets via code words.
OTRS's strength is in the helpdesk part of the equation. Once the perl requirements are met, OTRS is a rather stable helpdesk solution. Anything that can send an email can make a ticket in OTRS, or you can use SOAP integration and other methods to communicate directly with the database. OTRS can run on practically anything that can run Perl. It also has localization and ability to create your own themes/branding. OTRS is designed to go beyond Helpdesk into Change Management.
Which to choose? For helpdesk, I recommend OTRS. For hardware auditing, Spiceworks. Spiceworks may be all you need for a single file to download and install get-it-done application. OTRS scales better for larger organizations, is cross platform, and works on your database.