Which crypto currencies can merge mine with Bitcoin? I remember namecoin has the ability to merge mine with Bitcoin. Are there any others? Is this a bitcoin ability or ability of the coin that is merge minable?
Being able to merge mine with another cryptocurrency recquire that both chain use the same hashing algorithm, sha256 in our case, so in theory all those crypto could do it https://en.bitcoinwiki.org/wiki/List_of_SHA-256_crypto_currencies 

The most famous currently merge mining being Dogecoin with its parent blockchain Litecoin both using the same algorithm 

You can read more here: https://medium.com/altcoin-magazine/merged-mining-how-dual-mining-of-cryptocurrencies-works-98323ea57529