An online book that introduces digital filtering.
The Wikibooks DSP book is a good idea that never really seemed to come to fruition.
Practically a "Beginner's Encyclopedia of DSP", this book is a very comprehensive treatment of DSP fundamentals and also covers many related subjects. Like Understanding Digital Signal Processing, this book emphasizes an intuitive understanding of the subject, with a minimum of math. The full text is available online for free in both HTML and pdf format.