The Ethereum Wallet and the Mist Browser were the original desktop applications developed to allow users to manage their ETH and interact with the Ethereum network. While these projects have been officially discontinued, many people still possess old wallet files that may require these applications in order to open them.
Where to find older versions
If you are looking for an older version to recover your funds, you can still find the original files online. The official binaries for Windows, Mac, and Linux have been preserved on GitHub.
You can find releases ranging from version 0.2.6 up to the final 0.11.1 release. Having access to these older versions is often the key to successfully loading an older wallet that modern software no longer recognizes.