I know that this is a relatively new feature in gmod, but I am not able to grab a screenshot of it.
Currently it looks a bit plain except for the AN logo and colour scheme of course, and from reading
http://wiki.garrysmod.com/?title=Sv_loadingurl it looks as if you can do a lot with this.
For instance, you could show a summarised rule list, or how about an embedded video informing players how to install the latest wiremod - although this might be too much or it might not even work, another great idea would be a link to the donations page. Obviously you don't want too much on this linked page or it'll eat up the resources of connecting clients.