To scratch a personal itch, I created a small, easily browse-able database of NT/OT quotations – that is, places where the New Testament quotes the Old Testament. I pulled this together from existing sources I found online, just adding some usability:
Source code and database in SQLite and other forms - see the GitHub page
A simple web interface with filters and hyperlinked verses that bring up the text in the NET version in a popover, with links to the full context in netbible.org.
For similar efforts online, I found Bible Cross References from openbible.info, which has a much larger database of more general references and illusions. My page is limited to things that are direct quotations only.