No, those banner ads aren't some spyware that's snuck onto your browser. Sleuth has joined a banner exchange program in order to drum up some more traffic. We've been stuck at a fairly contant number of players for awhile now, and would like the community to expand.
Leave any complaints or comments you have about them here. In particular, I want to know whether anybody is having page layout problems because of them.
I'm still getting used to them myself.