[famfam]

Are they hits to the front page? Direct would suggest something like a Twitter client or email. But I don't see much mention of your url on Twitter. Perhaps it's been shortened. Really annoying that there's probably no way to figure out what's going on in that case. What's the network distribution of your visitors? Does it look legit?

[petercooper]

Direct would suggest something like a Twitter client or email.

Or a mention in a newspaper, magazine, or on TV. Even mentions in backwater publications or channels can have a big effect traffic wise.