My logic simulator of choice is Digital Works 126.96.36.199. It has a number of limitations, but it's free and you can make your own components fairly easily. The commercial parts aren't usable because they use IC package symbols that don't do anything to help convey circuit intent. It doesn't assign component designators, but you can add pin numbers to symbols you create (this deficiency occurred to me much too late).May be some one here can point to one thats more affordable
Schools seem to like Logisim because there's some automated test capability.