BiblicalTraining.org have made available a free online version of Systematic Theology by Louis Berkhof here. It is available in PDF or ePub format, or you can read it online by section.
This was the textbook for my basic theology courses when I attended Covenant Theological Seminary, and I highly recommend it. If you haven't yet purchased a copy for your own theological library, you can check it out for free to see if you would like to spend the money. Regardless, it is good to have a searchable copy available for use on your computer.
Thanks to Brandon Adams for letting me know about this.