my suggestions are untangle and WFilter.
If you have a spare computer with two nic cards and a lil bit of time after hours (this computer needs to sit between your firewall and your internet router/connection ) I would suggest trying this product:
http://www.untangle.com/
It reads like its is a fair application for what you are asking to do if it is just to see who is doing what on what website for how long...
now i will give you a brief introduction about WFilter.
if you wanna monitor your network the most convenient way is to monitor on a mirroring port of your switch.
what can you monitor with the help of WFilter?
1. keep a detailed record of each web Surfing and web posting.
2. record all incoming and outgoing email content and attachment.
3. monitor and archive instance messengers that contents and activities.
4.monitor and archive files transferred by web, ftp and IM tools.
5. implement a policy to filter internet access during working hours.
6. websites, messengers and p2p file downloading can be blocked to save bandwidth and raise productivity.
7. you only need to install WFilter in ONE computer to manage your whole network
You can download "Adblock" an ad blocking software for Google Chrome Directly from Google. If you are using Firefox you can download "Adblock Plus" from the site Mozilla Add-ons.
Many websites offer the download to the WinSPC software. The WinSPC website allows one to download the software. One can also download the software from CNET.
One can download Utility Billing Software by going to the official website of Black Mountain Software and doing a search for what Utility Billing Software on that website. Afterwards one can proceed to download it.
One can download the Unreal Media Server software from the 'umediaserver' website. One can also download this software from 'CNET' and from 'Softpedia'.
Research has shown that there are a number of sites where one can download Freemind software. This software can be downloaded for free from the CNET website as well as directly from the Freemind website.
One can download the TI Connect software by downloading from the TI website. It is a free download and the latest version of the software can be found there.
One can download torrents on the website "the Pirate Bay". Keep in mind that one is only allowed to download free available software or open source software on this website.
One can download the Microsoft DPM software from Microsoft's website. Evault and Technet also have the software available for download. Wikipedia has a page with information about the software.
One can find free web page design software available for download at the website for Microsoft Expression. It is a web design software and one can download a free trial from the website.
One can download Navicat software direct from the official Navicat software website. Alternatively, one can also download it from freeware sites such as Cnet and Run Patch.
One can download software on the internet. For example, one can go to the website of the Official Microsoft Download Center and download Internet Explorer 8 from the site.
One can find about the spam blocking software on a number of online pages. Wikipedia, Spam-filters review on 10TopTenReviews, and Spam fighter website.