A site explaining SQL indexing to developers—no crap about administration.

SQL indexing is the most effective tuning method—yet it is often neglected during development. Use The Index, Luke explains SQL indexing from grounds up and doesn't stop at ORM tools like Hibernate.

Use The Index, Luke is the free web-edition of SQL Performance Explained. If you like this site, consider getting the book. Also have a look at the shop for other cool stuff that supports this site.