Gallium makes aluminum soft like a wet tissue. Any other metals or alloys that are vulnerable to similar damage? Gallium can make aluminum soft and brittle as long as it bypasses the aluminum oxide normally formed on the surface of the aluminum (for example by scratching the surface). 

Mercury can weaken aluminum as well. 

Are there any other metals or alloys that can be "weakened" by coming in contact with gallium, mercury or any other substance?
Gallium and Mercury, the latter in particular are absolute pests and poisons to most metals. There are too many to mention that will suffer the same consequences. I will mention the few that don't. Iron, Platinum, Tungsten and Tantalum all tolerate them fairly well. There are certainly more examples, but the common rule is; most don't.