Litecoin is currently the leading alternative. It's got a shorter confirmation time, uses scrypt as opposed to sha256, and will in the end have a larger total of coins.
All these make it seem like a better alternative for micro-transactions in the end.
How can larger number of coins be an advantage (or disadvantage for that matter)? As long as the supply is fixed it doesn't matter if the total is just 1 coin or 100 trillion coins.
There's been debates running now in the Bitcoin community what the new 'denomination' should be. So far, it mBTC (millibitcoins) is leading. You can still write it as 0.001 BTC, or 1 mBTC.
I'm just wondering in terms of psychology, what impact it has to write in ever decreasing denominations? Pricing something as 0.000232 BTC is much more difficult to read and understand. So, if there are more coins in circulation, this would hopefully not happen until much later. It will inevitable happen (if it is also a success). But just for adoption and ease of use in the short and medium term it will be easier to read.
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.
Litecoin is getting a lot of traction. I had made a post about it that rose to the front page until it got mass flagged.
You can check out btc-e.com and see the absolutely massive price spike of litecoin. It went from .80c two weeks ago to trading at $4 consistently over the last week.