What is the longest known vanity address generated? Anyone can generate a vanity address of arbitrary length with appropriate tools, it's just a matter of performing enough computations. What is the longest known vanity address anyone has generated?
It's impossible to say for sure what's a vanity address and what's generated purely at random, but 1CFBdvaiZgZPTZERqnezAtDQJuGHKoHSzg is 34 characters with no digits in it other than the initial '1'.  That may well be a vanity address.  Then again on average 1 in every 515 34-character addresses you generate won't have any digits in them other than the leading 1, so possibly not.

The address with the longest initial list of lower-case letters is 1yaniraswqyghuJKCRrGwJUA2HakWtRad, and is clearly a vanity address since it appears in the same transaction as many other vanity addresses.

Edit: I just found this one:

1QBDLYTDFHHZAABYSKGKPWKLSXZWCCJQBX


It belongs to etotheipi, author of the Armory Bitcoin Client, and was found using vanitygen, as he said in a bitcointalk forum post:


  Yessir!  I got a little crazy with vanitygen and succeeded.  It
  should've taken about 70 days of computation time but I got lucky and
  found it in about a week...  (notice no digits either, only uppercase
  letters).
  
  Unfortunately, it's so cool that people don't even recognize it as a
  Bitcoin address :)