Icons are usually either square or have their own independent shapes, you rarely ever see rounded icon sets. However Flatro may be the best rounded icon theme for the Android platform.
The designer behind Flatro is actually the same designer who has designed one of my other favourite Android icon themes - Min. However unlike Min, Flatro features a lot more colour.
Every icon in the Flatro set is rounded and features a very flat design. Icons have blocky shadows, pastel colours and are very simple in terms of design detail. However the custom icons feature some very neat design trickery that gives icons a nice twist. For example in the screenshot below, the Gmail icon has the usually straight red lines, curved round the edge of the icon - it's a very nice touch.
Flatro features over 460 custom icons which may not be as many as some other themes out there, but it'll of course cover all Google stock apps and the most popular apps on the Play Store.