Gallium-based thermal compounds are commercially available, often called "liquid metal". See "Coollaboratory Liquid Ultra" for one example.

They're very effective, but must be used with extreme care because aluminum is highly soluble in liquid gallium. Aluminum is the most common material for heatsinks, and it will literally dissolve if the gallium touches it. Gallium is also electrically conductive, so if you accidentally dripped any into your processor socket I assume you're going to have a problem.

If you're careful with it and make sure to get a cooler with a copper contact area, it'll cool more effectively than traditional thermal pastes. I thought about trying it, but it seemed like more hassle than it was worth.