IEQ Pro 2.0 ($0.50 minimum) from EQ plug-in developer DDMF (ddmf.eu) is a multistage filter plug-in (VST/AU) with a few special tricks up its sleeve. It is easy to use and CPU efficient, and it sounds great. You can try it out by downloading the free demo, which drops out briefly every 30 seconds. You buy the full version through PayPal (you do not need a PayPal account), and, of course, the developer would appreciate more than the minimum PayPal processing fee.
You start by selecting a configuration (series or parallel), activating up to ten filter stages, and choosing a filter type for each stage. All the usual filter types are represented: lowpass, highpass, bandpass, allpass, peak, notch, and shelf. For instance, you might configure a channel strip EQ consisting of a low-shelf filter, one or more parametric filters (called AnaPeak and DigiPeak), and a high-shelf filter in series. Each stage has gain, width (Q), and cutoff- or center-frequency settings, although for some filter types, the gain setting is not relevant. You can enter values numerically or by dragging dots around the display. The response curves of the focused stage and the combined effect of all active filters are displayed with light and heavy traces, respectively.
Parallel setups differ from what you might expect: the output of all the filters is summed with the unfiltered input. Because the filters are not phase linear, this configuration produces interesting phase-cancellation effects. For example, start with two bandpass filters, high- and low-shelf filters, or bandpass and allpass filters and then dial in the color you like or automate one of the filter frequencies for sweeping phaser- and flangerlike effects (see Web Clip 1). IIEQ Pro could well become your go-to plug-in for these kinds of effects.