A (looks to be really old....

) Polk PSW-125 Subwoofer @ $55 from Kijiji. Wanted to add a sub to the 4 Edifiers I've got as surround speakers for a little bit of extra punch, but it didn't necessarily need to be audiophile quality since I only use it for gaming and windows audio (music plays through my receiver via optical).
The dude had posted pictures of it without a

for scale and I was shocked at how big it was when I picked it up (heh.... that line taken out of context would not be something to throw on a CV... ). It's 420X460mm and 420mm high (16.5X18").
Took me a while to figure out that I had to de-select the speakers as full range and set the crossover, but everything seems to be working as advertised now. (Just have to find a spot to put it in).
Polk Audio PSW125 subwoofer delivers powerful, accurate and musical bass at shockingly low prices. It features a 12-inch composite driver and a high-current amplifier for a deep, dynamic bass response.
www.polkaudio.com