Trafalgar Square, commemorating the Battle of Trafalgar in 1805, is perhaps the most famous of London's landmarks, second only to Big Ben. It's located at the crossroads of The Mall (the lane through St. James' Park to Buckingham Palace), Whitehall (leading to Westminster and the Houses of Parliament), and the Strand (leading to Fleet Street and the financial district). It's filled with statues and fountains and is always full of people, rain or shine.

statue near St. Martin in the Fields' Church, at the northeast corner
Nelson's Column
view from the south, the National Gallery on the left
one of several peripheral fountains
another peripheral fountain
yet another fountain
one more (yes, I really like fountains!)
monument to Charles I, on the south side
Admiralty Arch, entrance to The Mall