Gideon Greenspan recently posted on the Colored Coins dev list:

We developed this to help with debugging. but it might be of wider interest to people here, to see what others are up to:

http://coinsecrets.org/

It shows you all OP_RETURNs in the block chain, hex and ASCII, going back to the first one (block 228596).

Some of the more interesting ones:

http://coinsecrets.org/?to=287731.000001
http://coinsecrets.org/?to=271007.000000
http://coinsecrets.org/?to=268081.000000
http://coinsecrets.org/?to=268060.000002 (rick-rolling)
http://coinsecrets.org/?to=251768.000000
http://coinsecrets.org/?to=228596.000000 (first one ever)

See also:  Hidden surprises in the Bitcoinblockchain and how they are stored: Nelson Mandela, Wikileaks, photos, and Python software by Ken Shirriff