Proving Which Spam Filters work Best
Against viruses and spam. For obvious reasons - hackers and spammers put their efforts into circumnavigating the major systems since this will maximize the impact of their work. That's why smaller anti-suckware products will often do a better job since the focus isn't on them.
It leads to a sad but inevitable cycle of products being improved, gaining popularity, then losing their effectiveness since they are now a bigger target.
At least, until a watertight (rather than guess-work) solution is found. I believe this is impossible without changing the way email works at a fundamental level. Even the much praised challenge-response is subject to email spoofing.
Reminds me of why I like living in Australia - globally speaking we're relatively irrelevant, making us a relatively small target. Hopefully we'll stay relatively irrelevant, lol :p