When in the VAB or SPH, this mod pops up and reminds you to add certain things to you space craft, like a solar panel or ladders. Once you have put that part on you can check a small box and move onto better things.
Fourm: http://forum.kerbalspaceprogram.com/threads/85388-0-24-Pre-Flight-Checklist-v0-1
This will no longer be updated, vist the link above for full details