user1577: Bitcoin uses proof of work to secure the network, Ripple uses a global consensus system and PPCoin uses proof of stake.
Are there any known alternatives to the above methods?

user44734: I have been researching on blockchain for sometime now and some of them I found are a mix of some existing and some proposed. 

Proof-of-Stake 
Proof-of-Burn
Proof-of-Capacity 
Proof-of-Activity      
Proof-of-Checkpoint

Source: https://bytecoin.org/blog/proof-of-activity-proof-of-burn-proof-of-capacity/

user341: Another alternative is proof of burn.

user682: A really quick answer is this: The proof of work system is a solution to the distributed synchronization issue; in another guise it is called the Byzantine Generals' Problem. Thus, any solution to this problem is an acceptable alternative, however the proof-of-work solution is particularly suited to distributed systems.
You can read Satoshi Nakamoto's discussion of this here.

1577: <p>Bitcoin uses proof of work to secure the network, Ripple uses a global consensus system and PPCoin uses proof of stake.</p>

<p>Are there any known alternatives to the above methods?</p>
 44734: <p>I have been researching on blockchain for sometime now and some of them I found are a mix of some existing and some proposed. </p>

<ol>
<li>Proof-of-Stake </li>
<li>Proof-of-Burn</li>
<li>Proof-of-Capacity </li>
<li>Proof-of-Activity      </li>
<li>Proof-of-Checkpoint</li>
</ol>

<p>Source: <a href="https://bytecoin.org/blog/proof-of-activity-proof-of-burn-proof-of-capacity/" rel="nofollow noreferrer">https://bytecoin.org/blog/proof-of-activity-proof-of-burn-proof-of-capacity/</a></p>
 341: <p>Another alternative is <a href="https://en.bitcoin.it/wiki/Proof_of_burn" rel="noreferrer">proof of burn</a>.</p>
 682: <p>A really quick answer is this: The proof of work system is a solution to the distributed synchronization issue; in another guise it is called the <a href="http://en.wikipedia.org/wiki/Byzantine_fault_tolerance">Byzantine Generals' Problem</a>. Thus, any solution to this problem is an acceptable alternative, however the proof-of-work solution is particularly suited to distributed systems.</p>

<p>You can read Satoshi Nakamoto's discussion of this <a href="http://www.mail-archive.com/cryptography@metzdowd.com/msg09997.html">here</a>.</p>
