macbook pro lots of GB's used up by unknown and unidentifiable stuff??

I have a 2009 13inch macbook pro 2GB memory 2.26GHZ Intel Core 2 Duo running on software version 10.6.8 - and there seems to be approx 90GB being used that I can t account for or find.

Ive searched manually by "getting info" and by using the uilities?

The available GB seems to be declining rather quickly without me downloading anything or creating any large files (only really use Logic Pro 9 on it) etc...

I reinstalled the osx less than a year ago to start from scratch so to speak and have been quite conscious of what I do on it however I have just watched the available space go down without piling stuff onto the mac ( I use external hardrives to save things also )

Does anyone know how to locate what is using all this space or what/why it is happening?

Also of any good cleanup software that has worked for people?

Thanks