My favorite thus far has been "routersim".
I have used it time and time again. However, it is rather expensive... over US $200
http://www.routersim.com/

You can configure various cisco routers, switches and even setup hosts to troubleshoot connectivity and etc. There are "labs" that you work through, or you can use labs provided by a CCNA lab book... or you can make your own. The commands and look/feel of it is VERY realistic...

I went in on it with a couple of people when I was in school and we just made copies. We were too poor to buy our own copy at the time. It seems to be VERY popular on different torrent sites too...