I am looking for an a free piece of software written in php that will
run on a linux server. Open source is preferable. I want the software
to maintain a database of items much like the site
http://www.ratemyprofessors.com . I want users to be able to add
entries in different categories and be able to view them by category. I
would like each user to be able to vote in a similar manner, and I
want to customize the voting fields. A picture upload option would be
helpful but is not entirely necessary. I MUST be able to have more
than one voting category per item and the software should compute an
overall rating for each entry. Searchablity is also helpful. What is
out there? If nothing fits exactly what is
the closest match?

Hi
I recommend you Vote Pro ( http://www.vote-pro.com/ ).
It works on both Windows and Unix but it's not free or open source.
You can buy it from 29$ (single license). It's easy to use, skins
supported and has a nice back-end.
I think this software best suites you, though there are some others:
phpq (http://www.chumpsoft.com/products/phpq/), Vote Caster
(http://www.comdevweb.com/votecaster.php), MX Survey
(http://www.interaktonline.com/Products/Web-Applications/MXSurvey/Overview/),
X Poll (http://members.lycos.co.uk/xscripts03/).