[veritgo]

Evernote works great. It syncs to a web client which then syncs to native clients which are available on Mac, iPhone, and Windows.

Not as fast as Simplenote appears to be, but excellent nonetheless. Has saved me in a datacenter without my laptop many a time.

http://evernote.com/

[TJensen]

I'll second that. It is more feature-rich than simplenote, which I prefer: voice notes, pictures notes, etc. There is also a plugin for Firefox extension for grabbing clips. I (and several of my friends) are very happy customers.