Kerbal Space Program
Here’s something cool and fun for you budding rocket scientists… a space simulator that lets you build your own rockets and fly them to the moon. Be warned… even though has some cute critters in it (Kerbals), the rules of physics still apply! The program is (currently) free for download from kerbalspaceprogram.com
Here’s their video trailer, to give you a taste of the action…
Also, there is an excellent blog post that takes you through an entire mission… here.