HO Refugium

The ones ive seen most often use a pump to draw water into the refugium and an overflow simply allows water to spill back out, i've only seen a few tanks with them but none of them had excess bubbles.
The biggest downside of the HOB refugiums is that they dont provide the volume and space that a true sump can. One of the good things about them is they provide a natural pod nursery, where as in a sump the pump impeller kills the animals that pass through, the HOB allows pods to easily enter the main display tank.
They can be purchased fairly cheap (at least over here in the states) with all the components, whereas you would still need lighting and a pump for that one.

even though you don't get the benefits of added water volume, it is nice to at least have a place to grow chaeto and stuff a heater without fouling up the display area..