If you, like me, use a PC as your source, you might find the following useful (I haven't tried it yet, as I need a passive x'over for my purposes). I suppose a lot of folks have old PCs lying around and this seems like a good way to put them to use, without having to resort to buying dedicated digital crossovers.

I'm sure I've seen a couple of other packages out there too.

• Foobar 2000 crossover plugin

• AudioLens