For Barcelona, I'm now pretty reasonably sure that I will be staying in a hostel for the 3 months I am there (assuming I choose Barcelona over Madrid or Sevilla). There are two options available as I see it:
Downtown Paraiso Hostel which has prices from 17 EUR ($23) per night in the high season or about 510 EUR / $700 US per month. Free wireless internet (so crucial) and a kitchen available to use (like I'd cook)! This isn't the hostel I am staying at when I am in Barcelona, but I am going to try to make a visit to it at some point to see if it will suffice. I am most concerned with finding a spot to plop down a laptop + 20" external monitor with sufficient plugs. This is going to be the biggest issue with any hostel I try to stay in. Also has a roof terrace.
Kabul Barcelona Hostel is the other possibility. This is where I am staying while I am in Barcelona during this trip. Also includes free wifi internet as well as free breakfast that is likely to not be the greatest thing ever. Has a rooftop sundeck. Not sure of the exact prices, but I think it will be more expensive than the other hostel though, so it is less appealing. They had an email link for info about apartments so I shot one off to them to see what I got back. Will update in the future.
Edit: Bondi Beach has no good hostel options so I'll be getting an apartment in Australia.