Thanks to my Keywords facility, search engine bots spend hours and hours trawling thousands upon thousands of unique URL's around my website. This facility is supposedly like your ordinary tag cloud but unlike 99% of other websites out there, I decided to allow multiple keywords to be used concurrently.
Each page on my site can be tagged with multiple keywords, and it's down to my discretion how I choose to do it - I'll usually select around five tags for a page. All these tags are collated and shown as links on the Keywords page. However, each keyword is shown only if there are more than three pages referencing it.
So each keyword leads to a minimum of one unique URL, but due to allowing combinations of keywords, the number of URL's is exponential. Couple this with the fact that keywords can be selected so as to ignore pages, you can double that again (mathematics isn't my forte).
I'd love to be able to tell all the search engine bots to ignore my keywords, save your time and energy, you'll never get to see them all, but alas they're too stupid*. So it always amuses me on the rare occasion that I use Google's Webmaster Tools, to see that Google has trawled, for example, 58000 URL's on my site in February this year.
*EDIT: I just discovered that I can at least tell Google to ignore my keywords pages but now I'm not so sure I want to.