If you want to invest in a market-weighted basket of the top twenty…

If you want to invest in a market-weighted basket of the top twenty cryptocurrencies, there's now a Bitshares smartcoin that allows you to do so in decentralized fashion.

http://bittwenty.com/bit20.php

You can read about the details for how Bitshares smartcoins work here:

https://bitshares.org/technology/price-stable-cryptocurrencies/