If you use Google Analytics to track visitors then you might be interested in this.
http://priteshpatel.me/how-to-exclude-traffic-from-semalt-com-in-google-analytics-using-a-simple-filter/ (http://priteshpatel.me/how-to-exclude-traffic-from-semalt-com-in-google-analytics-using-a-simple-filter/)
Over the last few weeks I have been seeing this and wondered what it was. I thought it was a search engine crawler, it isn't, and I have added the above code so it doesn't show up in the stats and so giving false readings for visitors.