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http://coinmarketcap.com/ has estimates of market capitalization based on exchanges for various cryptocurrencies. A reasonable proxy for popular in some cases.

Litecoin is #2 but when you click the bottom link "Looking for Ripple?", Ripple is #2.

Ripple/Opencoin has 10x the market cap of Litecoin: http://coinmarketcap.com/ripple.html

But Ripple isn't a typical open source "crypto-currency". It's well financed and backed by big SF investors with the intention of making the founders/investors money, instead of miners, as the currency scales in price.

The reason to mine Litecoin in this context is because the algorithm (scrypt) hasn't been taken over by ASICs yet. So CPUs have a smaller disadvantage.

Last I checked, Litecoin was the only alternative currency that's been gaining serious traction over time.