user78: https://en.bitcoin.it/wiki/Bitcoin-Powered_Database
As far I know, the Bitcoin blockchain is pretty much the only data structure that is both global and tamper-proof.
global - There is one global instance of the blockchain (up to forks ... which are then resolved).
tamper-proof - If a block enters the blockchain, after 6+ confirmations this block can't feasibly be faked or altered.
A Bitcoin-powered database would be an API over the blockchain, that would expose a subset of the usual CRUD database operations. In fact, it would be an append-only data structure, because nothing can ever be really deleted from the blockchain - so only CREATE and READ operations will be implemented. Object Versioning will be used to emulate updates and deletions.
Have there been previous attempts to create such a database? I have seen the MerkleWeb proposal, but I haven't really understood it, so I'm not sure if it answers my question.

user1579: Namecoin is more or less what you're asking for.

user78: This is it - the CryptoSphere.
It uses some Bitcoin concepts, and might support paying for storage using Bitcoins. I believe that "merge-mining it" using the Bitcoin blockchain would be possible (it's too soon to tell, the design is preliminary).

78: <p><a href="https://en.bitcoin.it/wiki/Bitcoin-Powered_Database">https://en.bitcoin.it/wiki/Bitcoin-Powered_Database</a></p>

<p>As far I know, the Bitcoin blockchain is pretty much the only data structure that is both global and tamper-proof.</p>

<p><strong>global</strong> - There is one global instance of the blockchain (up to forks ... which are then resolved).
<strong>tamper-proof</strong> - If a block enters the blockchain, after 6+ confirmations this block can't feasibly be faked or altered.</p>

<p>A Bitcoin-powered database would be an API over the blockchain, that would expose a subset of the usual <a href="http://en.wikipedia.org/wiki/Crud">CRUD</a> database operations. In fact, it would be an append-only data structure, because nothing can ever be really deleted from the blockchain - so only CREATE and READ operations will be implemented. Object Versioning will be used to emulate updates and deletions.</p>

<p>Have there been previous attempts to create such a database? I have seen the <a href="https://bitcointalk.org/index.php?topic=53062.0">MerkleWeb</a> proposal, but I haven't really understood it, so I'm not sure if it answers my question.</p>
 1579: <p>Namecoin is more or less what you're asking for.</p>
 78: <p>This is it - the <a href="https://github.com/tarcieri/cryptosphere" rel="nofollow">CryptoSphere</a>.</p>

<p>It uses some Bitcoin concepts, and might support paying for storage using Bitcoins. I believe that "merge-mining it" using the Bitcoin blockchain would be possible (it's too soon to tell, the design is preliminary).</p>
