In addition to the corner bass traps, I have made some broadband absorbers for my home recording studio. To make these, I used a 2' x 3' frame of 1"x4" lumber, painted black, along with a single panel of 2" Owens Corning 703 Rigid fiberglass insulation. For the broadband traps, I chose to use some burlap coffee bags to cover the insulation to add a different and interesting look, and inserted these into the painted frames. These panels hand easily from some screws in the wall, and were placed along the side walls of the mix position and behind the monitors. Additionally, I have one broadband panel mounted to the ceiling between the monitors and the mix position. Remember, all flat hard surfaces produce reflections across the frequency spectrum, and these broadband absorbers are effective down to around 200 Hz.
For some additional reduction in mid to high frequencies and flutter echo, I have made some different treatments for the rear of the room. Again I have used 1" x 4" lumber frames measuring 16" x 32" and covered in burlap. I stuffed these panels with regular pink R-13 insulation, with the paper side to the wall. These rolls of insulation can be found at most big box hardware stores for cheap. When compared with acoustic foam panels, the effectiveness of regular insulation holds up quite well. Check out these graphs from F. Alton Everest's "Master Handbook of Acoustics", comparing the absorption of foam(Fig 9-12) vs. fiberglass insulation.
Along the back wall of my room, I have a 6' x3' diffuser/absorber panel to go along with the corner bass traps. This simple treatment is quite cost effective, at less than $30. You can check out more about this particular homemade treatment and plans at thedawstudio.com