Namecoin (<a href="http:&#x2F;&#x2F;namecoin.org" rel="nofollow">http:&#x2F;&#x2F;namecoin.org</a>) gives you 520 bytes out-of-the-box. It was designed to store arbitrary data on a blockchain. Being merge-mined with Bitcoin, it enjoys the highest security against a 51% attack of any altcoin.