For the younger folk I would suggest:
Sophie's World - Jostein Gaarder
Philosophy of Science - Geoffrey Gorham
And my personal favourite, although pretty hectic (especially if you're a bit behind on philosophy like myself)
Goedel, Escher, Bach - An Eternal Golden Braid - Douglas Hofstadter
And a painful introduction to language
Tractatus Logico-Philosophicus - Ludwig Wittgenstein
A bit of art-philosophy mixed with brilliant use of language and a murder mystery
My Name is Red - Orhan Pamuk
Also Ayn Rand is very good, I'd suggest Atlas Shrugged or The Fountainhead
But to be honest there is so much out there, I suggest spending a good hour or two in a bookstore at their philosophy and/or science section and just go with what you like. From there on you can always go on to bigger things. Wikipedia as has been suggested earlier is also an excellent place to start and just go link-hopping from there.