If you've spent any significant amount of time filling your plug-in folder with effects, chances are, you've probably run into a multi-band effect or two. If you're not familiar with the concept, a multi-band effect divides the signal to be effected into separate frequency ranges (or "bands"), allowing you to assign completely different settings for each of these bands. This is most often used for compression effects (especially when mastering), allowing you to compress, for example, your bass frequencies a bit tighter than your mids or highs, or vice-versa. Pretty cool, right? The problem is, you generally only find multi-band effects to come in compression and distortion flavors. What if you wanted to make any plug-in in your folder multi-band capable? Or even crazier, what if you wanted to have completely separate effects on each frequency band? If your DAW has effects sends (and most do), this is surprisingly easy to do.
The first step is to figure out how many bands you really need. You are really only restricted by how many sends you have available through your DAW, but practically speaking, you'll want to keep this number as low as you can just to keep things simple. Once you've done this, go ahead and set up the necessary amount of sends on your channel and set the send level to 100%. You'll want to turn off the output of the main channel too, since you're bussing the signal to the sends.
The next step is to figure out what frequency ranges you want each send to cover. Once you've done that, assign an EQ to each send. You'll want a rather precise EQ for this purpose, as we're just using it to restrict the signal on each send to a specific band of frequencies. Look for a Linear Phase or Mastering EQ for a good start. Now, you'll want to go into the EQ on each send and define the frequency bands by using lowpass and hipass filters to set the high and low range of the frequency band for each send. For best results, you'll want to make sure these don't overlap very much, if at all, or you may run into some phasing problems.
Once you've divided each send into its own frequency band, the sky is the limit. You can either assign multiple copies of the same type of effect after the EQ on each send and tweak the settings individually to taste, or even assign completely different effects for more extreme and experimental results. Like the way one distortion sounds on the low end, but prefer a different one for higher frequencies? No problem! Want to have a sound where only high frequency sounds are sent to a delay? You can do that too. Want to send only the mid-range frequencies through a filter? Piece of cake. The sky is literally the limit here and you can get as complex and involved as you like. As with many things, simplicity usually yields the most useful results, but don't let that stop you from experimenting and trying out some crazy things.