spindown: Conversion of ordinary currency to and from Bitcoin can be a hassle. The most convenient way is to use online exchanges like Mt.Gox, but they require detailed personal information (scans of your photo ID, proof of residency) and manual verification before you are allowed to trade, which can take time.
Coinomat.com is probably the easiest way to exchange currencies and money. It doesn't require any proof (far as I can tell). You do pay for that convenience in worse rates.
As far as other cryptocurrencies go, Litecoin is the most popular after Bitcoin. The thing is, alternative cryptocurrencies tend to be highly correlated with Bitcoin (and are even harder to buy and sell), so you might as well stick with Bitcoin unless you have a specific reason to think that a currency might be a good buy.
The correlation of alternate cryptocurrencies and bitcoin isn't that high. Sure, the dollar value changes with the fluctuations of the bitcoin, but the ratio with the bitcoin also changes. Also as I mentioned there are some reasons why alternate currencies (in particular litecoin) could become more valuable.
Mr.Caine: Invest in Dogecoin today and get rich.
Just don't. I tried hitching a ride on a new coin, they go up quickly, then go down and stay down. They're good for very early adopters and miners, but you need to get out of them fast. There's really very little to distinguish them. That said, if you want to get seriously into the market, then hitching a ride on new currencies is a way. I still won't recommend it, because while I think that bitcoin and litecoin will continue to go up, and that they will drag other currencies with them, it's IMO too risky to invest in alternate coins.