open source web filters/proxy?

I am in the process of researching different types of proxy/web filters. It would be for a small business environment to filter all web traffic- about 100 users. I would like the ability to setup different user tiers and rules all based on a whitelist. I have been looking into free/cheap but would like some insite on what other people are using.

Have yall' ever used: dansguardian or smoothwall?

I an a n00b at linux, but would love to learn and this maybe a good opportunity to practice, if it is required!
Cheers
Sitizn Wille