Good news! LEGO is giving a piece of software away for free.
If you’ve lost the CD containing the graphical NXT-G programming environment that came with your LEGO MINDSTORMS NXT set, you can now download a perfectly official and legal copy of the retail NXT-G 2.0 LEGO website. This is not the education version of the software, but can still be used with the education MINDSTORMS sets.
If you only have an older version of LEGO MINDSTORMS (RIS or earlier versions of NXT), or you simply want to play around with the software, I would recommend you grab a copy as this is a piece of free software that has not been available in the past.