A Visual Introduction to DSP for SDR
visual-dsp.switchb.org
visual-dsp.switchb.org
The section on FIR, IIR, and how to make high-pass and low-pass filters was also useful to me. I have a decent math background, so I'm finding one of the hardest parts of working through an intro communications textbook is figuring out the EE jargon (in-phase and quadrature, narrowband and wideband, etc).
I highly recommend whole Cyberspectrum series, or attending live if you are in the area.