Today, Ethereum joined the Bitcoin and Ripple in the $100billion+ market cap club. This is even more remarkable for a Cryptocurrency which was valued around $0.9 billion this time last year: an ~11,000% rise in value.
Source: Coinmarketcap.com on 6th January 2018
The value of Ethereum is still in a speculative stage of price discovery. There has never been an asset like this, and markets are playing with demand to see what price is stable. It is unique in that a large number of the other Cryptocurrency tokens are ERC-20 assets based on Ethereum itself, so it derives demand not just from itself, but from the success of a large number of competing projects. Nevertheless, it is still an interesting exercise to compare it directly to the market capitalisation of leading companies in the US S&P 500.