Which metals are liquid at room temperature?

Alongside mercury, caesium, francium and gallium may all be liquids at room temperature.
Being very dense (metals), horseshoes, bricks and even cannonballs theoretically may float in these liquids.

   Caesium (Cs) was discovered in 1860 by Robert Bunsen using the spectroscope he had invented with Gustav Kirchoff (the man who discovered that signals travel down telegraph wires at light speed). It wields the atomic number 55 and is mostly used in atomic clocks (used to define the atomic second). It also explodes very violently when it comes in contact with water, even at -116 degrees centigrade (−177 °F). The name Caesium is derived from the Latin word caesius, meaning 'sky blue' due to the bright blue lines it produces as part of its spectrum.

   Francium (Fr), element 87 on the periodic table, is one of the rarest elements known to man. There is a recorded ever 30 grams of it present on Earth. This is due to it's radioactivity, it quickly decays into other, more stable elements (so it's a liquid metal, but just for a few seconds at most!). It's the last element to be discovered in nature and was isolated in 1939 by Marguerite Perey at the Curie Institute in Paris. Francium melts at 27°C (81°F) and boils at 677°C (1251°F).

   Gallium (GA) was discovered by french chemist Lecoq de Boisbaudran in 1875. It was the first new element to confirm Dmitri Medeleev's Prediction of the periodic table. It is mostly used in microchips because of its atypical electronic properties. CD players also make use of this metal because, when mixed with arsenic, converts an electric current directly into Laser Light, which is used to 'read' the data from the discs.
   These metals are liquids at such low temperatures due to the arrangement of the electrons in their atoms. The arrangement makes it difficult for them to get close enough to form a crystalline lattice. Each individual atom flows freely without attracting to each other, exactly the behaviour in other liquids such as water.