They all have their own qualities, but you could do this with, for example, the EQIII that comes with Pro Tools.
You'll need to figure out what dB/octave is best for you, but say you want to crossover at 100Hz and 1K Hz:
You'll need to set up three output pairs.
Output pair 1: Low freq. Use a Low Pass Filter set to 100Hz
Output pair 2: Mid freq. Use a Hi Pass filter set to 100Hz and a Low Pass filter set to 1K Hz
Output Pair 3: Hi freq. Use a Hi Pass filter set to 1KHz
Most DAW's should already come with the plugin you need to do this. All that's left is to assign each pair of tracks to separate outputs, and voila, you've made an active crossover!
For general purpose, I suggest 24 dB/octave as a starting point.