- NANDLAND has a good VHDL reference and a set of tutorials on desiging various useful circuits.
- VHDLwhiz has a set of free tutorials. The author takes a somewhat unconventional approach starting with testbenches rather than synthesizable code, but he generally explains with clarity and detail.
- Simple but useful collection of resources from Jayram Moorkanikara at UC Irvine. This is the closest thing to a usable language reference that I've found.
- VHDL Primer from Jan Van der Spiegel at Penn. A few things are outdated, but it's a good introduction to VHDL.
- Free-Range VHDL -- This free book is one of the most concise and helpful introductions to VHDL I've found.
Written by Bryan Mealy and Fabrizio Tappero of Free Range Factory, distributed under CC-BY-SA.
- Ricardo Jasinski, Effective Coding with VHDL: Principles and Best Practice -- This is an excellent book that describes how to do things correctly with VHDL. It goes through each feature of the language and describes not only how to use it, but how to use it effectively for clean and maintainable code.