I can't type well in Hebrew, apologies in advance.

You forgot to mention one of the classic sci.math cranks: Archimedes Plutonium. He dealt, for a few years, with the classic 0.9999 question, mixing in issues like p-adic numberc, etc. etc. You also forgot to mention another tempting crank-magnet: since Godel proved we cannot be sure math [ZFC, number theory or what have you] is consistent, numerous cranks have set out to prove it isn't. They are even more amusing than the garden variety. Note that not only non-professionals come across as cranks: there have been professional mathematicians in that position. A recent example is some story in recursion theory, but I do not remember the details.