user12835: People have been talking a lot about side chains, but I thought this has been around for a while under a different name. Coins like Namecoin, Devcoin, ixcoin, i0coin, groupcoin, etc are all merge mined and have existed long before I ever heard the term "Side Chain". 
Am I correct in my assumption that side chains are just a rose by another name (previously merge mined coins)? Or is there something I am missing here?

user192: Sidechains are chains which allow 2-way peg, that is, moving bitcoins to the sidechain and then back to the main chain without trusting any third party, like you do, for example, when you move Bitcoins to say, Ripple (there you have to trust Bitstamp or another gateway). As such, there's no sidechain in existence, simply because the technology has not been implemented yet.
2-way peg is orthogonal to merged mining (you can do each of them separately or both). It is generally assumed that a sidechain would have merged mining because the bitcoin wizards designing sidechains accept that merged mining is generally desirable because of its superior security properties.
Compared to atomic swapping, 2 way peg is much slower but it doesn't require a counterpart that wants to move coins in the other direction.
So when sidechains are implemented, it is likely that most of the transfers are done with atomic swapping and only the first movements are done with 2 way peg, which is mostly there as a last resort in case you can't find someone else that wants to move the same quantity in the opposite direction.
Here are more details on how 2-way peg works:
How is a side-chain merging back to Bitcoin chain protected against double-spend? Where are the side-chain mining fees?

user14233: A side chain is a blockchain that runs parallel to the main bitcoin blockchain and can synchronize with it directly, allowing decentralized value transfer between the two chains. A user is able to essentially lock up a certain amount of coin on the main bitcoin block chain which then frees up an equivalent amount of coin on the side chain. This side chain may be either a sidechain compatible altcoin or just some test ground for new Bitcoin features. The coin that was transferred to the side chain can be returned back to the main bitcoin blockchain at any time. The value transferred back will be the same as the value transferred out.

12835: <p>People have been talking a lot about side chains, but I thought this has been around for a while under a different name. Coins like Namecoin, Devcoin, ixcoin, i0coin, groupcoin, etc are all merge mined and have existed long before I ever heard the term "Side Chain". </p>

<p>Am I correct in my assumption that side chains are just a rose by another name (previously merge mined coins)? Or is there something I am missing here?</p>
 192: <p>Sidechains are chains which allow 2-way peg, that is, moving bitcoins to the sidechain and then back to the main chain without trusting any third party, like you do, for example, when you move Bitcoins to say, Ripple (there you have to trust Bitstamp or another gateway). As such, there's no sidechain in existence, simply because the technology has not been implemented yet.</p>

<p>2-way peg is orthogonal to merged mining (you can do each of them separately or both). It is generally assumed that a sidechain would have merged mining because the bitcoin wizards designing sidechains accept that merged mining is generally desirable because of its superior security properties.</p>

<p>Compared to atomic swapping, 2 way peg is much slower but it doesn't require a counterpart that wants to move coins in the other direction.
So when sidechains are implemented, it is likely that most of the transfers are done with atomic swapping and only the first movements are done with 2 way peg, which is mostly there as a last resort in case you can't find someone else that wants to move the same quantity in the opposite direction.</p>

<p>Here are more details on how 2-way peg works:</p>

<p><a href="https://bitcoin.stackexchange.com/questions/24549/how-is-a-side-chain-merging-back-to-bitcoin-chain-protected-against-double-spend">How is a side-chain merging back to Bitcoin chain protected against double-spend? Where are the side-chain mining fees?</a></p>
 14233: <p>A side chain is a blockchain that runs parallel to the main bitcoin blockchain and can synchronize with it directly, allowing decentralized value transfer between the two chains. A user is able to essentially lock up a certain amount of coin on the main bitcoin block chain which then frees up an equivalent amount of coin on the side chain. This side chain may be either a sidechain compatible altcoin or just some test ground for new Bitcoin features. The coin that was transferred to the side chain can be returned back to the main bitcoin blockchain at any time. The value transferred back will be the same as the value transferred out.</p>
