I learned about this lovely little program when I got my lap top and didn't want to pay for a second copy of Final Draft Pro (which I had on my PC tower.) And let me just say...I think I like writing on Celtx more. It doesn't have the application where robot voices can read your script, but there are a lot of other things you can do with it.
I actually really adore the index cards application; where you can write your scenes and characters on digital cards and swap them around to see what scenes would look like in a different order.
Celtx isn't only for screenwriting either. You can created many different projects ranging from film, audio, theater, av and yes, even comic books.
It's freeware and the website also allows you to upload your projects to share with others.
Honestly, it doesn't matter what program you use, as long as its in the correct format, so be sure to have a source on handy to double check yourself before you submit anything to a contest.
I like Syd Field's Screenplay or How Not To Write A Screenplay by Denny Martin Finn, two books I bought for my screenwriting course about a year ago.
Just remember to have fun with it!