What is the largest performed/possible bruteforce attack to date? I've read that cracking 128-bit key is currently out of reach of all humanity. However, I can't seem to find any information on what scope of brute force attacks have been performed or are possible at the current time. Can someone provide some information on this subject with quotable reference?
The last major effort I know of for cracking keys was the Distributed.net effort.

You can find the project page at http://www.distributed.net/RC5/en. In 2002, they cracked a 64-bit RC5 key using at total of 331,252 computers over 1,757 days. Their maximum throughput was "equivalent to 32,504 800MHz Apple PowerBook G4 laptops or 45,998 2GHz AMD Athlon XP machines or (to use some rc5-56 numbers) nearly a half million Pentium Pro 200s." as it said in their press release.

At the time, they moved on to doing a 72-bit key, which would be 256 times harder. 

If we do some Moore-Law-type hand waving, we'd see a $2^7$ (128×) increase in compute power in ten years. That means that if you started today, it would only be twice as hard as breaking 64-bits was a decade ago. Doable, but not many people would actually want to bother.

Even if you're a government, buying a zero-day exploit and tailoring your own malware would be cheaper.