Here are a list of some of the free online resources I use on a regular basis in addition to Bibleworks on my PC.
The works of Thomas Brooks and Thomas Watson are the best.
Almost all writings from the 17th to 19th century are available online in one form or another.
Go to search tools and change it to the 19th century to download books for free.
The “International Critical Commentary” is one example of a great resource.
Always preview books as much as you can through Amazon and Google before you buy them.
The commentaries of John Calvin, John Gill, Matthew Henry, Albert Barnes, Matthew Poole, John Lightfoot, A. T. Robertson, and the sermons of Charles Spurgeon are all available online for free.
Bible Study Tools:
http://tsk-online.com/ – The Treasury of Scripture Knowledge which is a great reference tool.