I'm currently researching what malicious activities can one do with a computer containing a high-end GPU. So far the only uses I found were Bitcoin mining and password cracking. Are there any other possible malicious uses for such computers?
A high-end GPU makes it very easy to do any sort of computational process that can be highly parallelized, each individual process doesn't need a lot of memory, and requires similar operations to be done to differing starting input.
So if you have md5 hashes with salts that you want to find the password that goes with, its trivial; e.g., generate a simple scheme to feed the GPU billions of passwords per second to try simultaneously and compare to the hashes you have. You could also do things like try constructing hash rainbow tables as well.
Another good candidate would be brute-forcing decryption of a file that's encrypted with a strong cipher. A GPU could plausibly be used to parallelize the process of doing trial decryption under many candidate passphrases.
I wouldn't say bitcoin mining is malicious; its basically the intended method of generating currency (except a GPU makes it more efficient).
Off the top of my head could also use a GPU to better crack simple CAPTCHAs using computer vision techniques.