In a previous blog post, I recommended a book named "Tithing: Low-Realm, Obsolete & Defunct" which explains the truth about tithing. I forgot that it's available online to be read for free. It's a great little book which not only explains how the Old Testament law of tithing has been dressed up and falsely taught to New Testament believers but it contains some wonderful truths about our position as sons and daughters. To read this book online, follow this link.
Joel has also posted a blog with some additional information about tithing which I think you'll find helpful.