- VHDL language reference by Michael Riegert. This is a cleaned-up version of an older guide, with some updates for VHDL-2008.
- VHDL-93 language reference from HDL works. This is a clean and complete VHDL reference guide, but only covers VHDL-93.
- Simple but useful collection of resources from Jayram Moorkanikara at UC Irvine, including a helpful language reference for VHDL-93.
- 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.
- VHDL Primer from Jan Van der Spiegel at Penn. A few things are outdated, but it's a good introduction to VHDL.
- FPGA Tutorial blog with a series of articles on key big topics in FPGA/VHDL design.
- EDA playground is an online editor where you can run VHDL code with a number of simulators.
- 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.