A tea clipper. Also nearby is the Gypsy Moth, which takes much less time to walk around but is I think equally impressive (if not in scale then in significance). There did also use to be a small book-boat opposite the playground, just to the left as you look across the river.
Also the name of a pub - Cutty Sark Tavern
Read anything about the Cutty Sark at it's offical website.