A couple of things to add:
TI's Opamps for Everyone goes into great detail on various opamp circuits and their design criteria.
If you have a mind for designing your own filters, GNU Octave's signal and controls packages more or less replicate commercial Matlab's controls and signal processing toolboxes (console mode only, of course).
LTSpice is a free SPICE with built-in schematic capture. It is very much tuned for selling Linear's catalog, since it contains models for many of their parts, but is also generally useful.