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Cutty Sark -- or at least the rebuilt version. This is at the Thames riverside, on the Greenwich side. On the left side of the photo you can see a little brick building with a domed roof: that's the entrance to the tunnel under the Thames.
In front of the gates of the Royal Observatory in Greenwich. The prime meridian runs just behind the gate on the left (where you can only get to when you pay the admission -- well worth it for the exhibitions IMO).
In the front, you can see the stands that were built for the Olympic Games: the equestrian competitions were in Greenwich Park. They were taking them down, but the complex is so big that it's quite a bit of work.
To the right, you can see the O2 Arena in the distance. Straight in the middle, you can see the high buildings of Canary Wharf.
On the Thames Walk, which runs from Canary Wharf to the Tower, along the north side of the river. You can see it's low tide here: the difference in water levels can be up to 9 meters.
Further along the Thames Walk.