[wladimir]

Two promising open source cryptocurrencies that are not bitcoin forks are Ripple and Opentransactions. These can also be used on top of Bitcoin, or the others, to provide some functionality (such as decentralized exchanges) that Bitcoin itself has not.

As for a big impact on the value of bitcoin, I don't think so. Bitcoin has the networking effects for now, many people are working on accepting them. It may be that the additional cost of accepting, for example LTC is lower when already accepting BTC, but BTC has a large first mover advantage.