Today two metals adjacent in the sixth period of the Periodic Table, rhenium and osmium, and in common with their neighbours, iridium, tungsten and platinum, they are very dense. In fact, osmium is the most dense element of all, narrowly pipping iridium.
Osmium has few uses as it is rare and hard to work with. In fact its compounds are so smelly that its name is derived from the Greek ‘osme’ meaning smell. (The word anosmia, meaning loss of the sense of smell, has become well known just recently.) But osmium is extremely hard wearing and found use, often alloyed with iridium, in high quality nib tips for fountain pens, as well as in electrical contacts and gramophone styluses. One fountain pen manufacturer even named their company, Osmiroid, after the alloy. They were once well known, and I believe I had one of their calligraphy sets as a child, but as is the fate of many successful companies they were bought out and the name disappeared.
The major use of rhenium is as an alloy with nickel and other metals in the turbine blades of jet engines (though I acknowledge my design looks more like the titanium fan blades at the front of the engine than the internal turbine blades). These blades must withstand temperatures of 1500 °C or higher and be incredibly strong. Most metals would melt or deform at these temperatures but rhenium, which has a very high melting point itself, increases the melting temperature of the alloy significantly. Amazingly, each blade is cast from a single crystal. This further raises the heat tolerance, ensures that there are no fault lines to weaken them and renders them more resistant to corrosion. Fortunately just 3% rhenium content is all that is needed as rhenium is rare; hence old turbine blades are recycled to reclaim and reuse the rhenium.