What fonts work best on-screen? An in-depth comparison of Droid Sans, Helvetica Neue (iOS) and Segoe UI (Windows 7). Summary: it's Droid Sans (Ian Hex, 2400 words).
The successor to Droid Sans on Ice Cream Sandwich is Roboto. Roboto's designer Christian Robertson gets right of reply to Stephen Coles' critique (4000 words).