I recently was referred to this great tool from Logos Software. The way it works is that you add their code to your blog’s template and when you encounter a Bible passage like John 3:16, the blogging software pops up the Scripture reference.
Hover your mouse over the reference above and see what happens.
Update (2012): I had so many problems with Logos Software’s RefTagger, I moved to another product, bib.ly by John Dyer. It’s a great product!