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Along with Google Analytics, I've used reinvigorate (reinvigorate.net). It's given me some useful information such as search terms used in search engines that got them to my site.
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Do you have your own search engine on your site? You'll probably have some logs somewhere that will show you what people are searching for. Google Trends is also a great way to see what words people are using.
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Spamato may be of interest to those that are looking to have spam filtering at the desktop level. It can be used with various e-mail clients, including Thunderbird and Outlook and uses a few different methods for filtering out the spam (see the site for specifics). I'm using it at home and it has done a very good job (much better than Thunderbird
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Also be sure to use the language and terms that the people using your services use.