To expand on
@Gt403cyl 's commentary:
The first liquid metal (LM) product was actually CoolLaboratory Liquid Pro (CLP). It's a runny mess of metal that's liquid at room temperature. Gallium is one of its main ingredients. It can expose aluminum to oxidation, causing it to corrode rapidly (normally aluminum is protected by a thin layer of aluminum oxide; gallium acts as a catalyst by allowing oxygen to breach that layer and fully oxidize solid aluminum). It has a thermal conductivity significantly higher than non-metal TIM, but still short of indium solder.
Later CoolLaboratory introduced Liquid Ultra (CLU). It is a reblended product similar to CLP, only it has some kind of a carbon/graphite matrix that makes it more paste-like at room temperature. It's still difficult to work with, but not so difficult as CLP. Most reviewers generally accept that CLU and CLP have similar thermal conductivity. I've seen more than a few tests that show CLU to be slightly better. It's also heavy on gallium, and it is known to "sink into" copper, essentially corrupting it by turning it an odd silvery shade. If you leave it in contact with two joined copper surfaces for any extended period of time, eventually it will turn into a crusty solder-like material that is very difficult to break. It's hard to get copper->copper bonds separated once CLU gets into the mix.
CoolLaboratories also has a "metal pad" product. It isn't as popular as their others (I don't think) and isn't often discussed. Mostly it's intended for applications where thermal pads might be necessary instead of a paste. It has no gallium in it (according to the SDS, it's indium, copper, and bismuth) and it melts at 58C. It has to cycle once before it works.
Phobya released their own liquid metal product - Phobya Liquid Metal - that seems to behave similarly to CLU. It may be a tad worse, but it's much cheaper. It's also got gallium in it, so watch out around aluminum.
Then there's Thermal Grizzly Conductonaut. It's a lot like CLU, but it claims higher thermal conductivity. I haven't seen any independent testing to confirm TG's claims about Conductonaut, but the stuff certainly does work. It's my current TIM of choice. I don't think I've quite mastered application yet, though.