GNS3 http://www.gns3.net

So there is a Cisco router simulator called dynamips. I was using it for a while on linux. It is kind of hard to setup and a pain in some ways. Now there is a graphical front end to it:

http://www.gns3.net Graphical Cisco Router Topology emulator

It makes it super easy to lab up simple topologies virtually (no clunky router hardware to dig around with you), and to test configurations and syntax. All you need in gns3 and at least one IOS binary. I used it on OS X, and it worked like a dream. Just get the DMG, click the image, copy GNS to the applications directory, then in the config stage I pointed gns at some images I had on my laptop (my tftp library I keep to upload to the lab). It fired up.

Then you can drag and drop some topologies. Console into the routers, etc.

This site had some nice tutorials for gns3 that the gns3 site doesn't cover.