The keep of Conisbrough Castle, England
St. Mary, Great Leighs, EssexThis tower is Norman and the third largest in the country, with an internal diameter of 17' (5.2 m.). (See "The Round Church Towers of England" by Stephen Hart, pub. Lucas Books, 2003.) It has a W. doorway composed of two orders, the inner decorated with a roll moulding and the outer with chevron, while directly above there are two narrow round-headed windows below the bell-stage, and positioned around the tower are three of the slightest of buttresses, featuring re-used Roman brick. The bell-stage is pierced at the cardinal points only and the tower is now topped by an octagonal shingled spire dating from 1882.
St Peter & St Paul's Bardfield Saling, Essex.I don't know if that counts as a Buttress, although it appears to do the same job.
Broomfield and Lamarsh in Essex both have unsupported round towers, as do Pentlow and South Ockendon (but both with a castellated round tower)
Schwingen in Switzerland. I think this is an apse rather than a round tower but I couldn't resist the flying buttresses.
Drumcliff Round tower - illustrating the value of buttresses.
Beachamwell, Norfolk, Three-stage, round tower, the thicker wall at the base acting as a buttress.