Libronix 4 - Free!
Logos' Libronix has been my favorite Bible study software for years. They've just upgraded their engine to version 4, and although they don't advertise it on the front page, you can get this new engine for free. Of course, they'd like you to upgrade to a new version of one of their nice packages, but if you already have all the books you need, you don't have to. The engine comes in both a PC-version and not-yet-ready-for-prime-time Mac-version. If you want to see everything the new version can do, take a look here. If you're serious about studying your Bible, take a long, hard look at Libronix.