I have been looking for a small dsp unit to use on some projects where space is a consideration and I finally decided on minidsp’s balanced 2×4 device.
minidsp.com – balanced 2×4

I recently got to play with the non balanced version of the same device (RCA inputs and outputs) but wanted the balanced version to interface with my other balanced equipment. Out of the box, the unit doesn’t actually do anything. You have to purchase a “plugin” to give it a purpose. In my case, I selected minidsp’s most straight forward plugin, “2×4 Advanced” for $10. There are more elaborate plugins that can perform 4 way crossovers and such but I just needed some basic EQ out of the box. For a complete list of available plugins, this link explains them all.
minidsp.com – 2×4 plug-ins
Once you purchase a plugin and install it, the plugin becomes your interface for the device. Once you’ve connected to the device, you can change parameters in almost realtime via the USB connection. Once you’ve finished making your adjustments, you can store the configuration to the device and unplug it from the host computer. Then the device will stand on it’s own. This coming week I will be installing the 2×4 on a small portable sound system I use on my ballet gig to EQ the speakers. Here is a video that explains the minidsp concept:

Application notes for the 2×4 series include:
minidsp.com – stereo 2 way xover
minidsp.com – auto eq with REW software
minidsp.com – subwoofer integration with minidsp