  Half a decade after Bitcoin became the first widely used cryptocurrency,
blockchains are receiving considerable interest from industry and the research
community. Modern blockchains feature services such as name registration and
smart contracts. Some employ new forms of consensus, such as proof-of-stake
instead of proof-of-work. However, these blockchains are so far relatively
poorly investigated, despite the fact that they move considerable assets. In
this paper, we explore three representative, modern blockchains---Ethereum,
Namecoin, and Peercoin. Our focus is on the features that set them apart from
the pure currency use case of Bitcoin. We investigate the blockchains' activity
in terms of transactions and usage patterns, identifying some curiosities in
the process. For Ethereum, we are mostly interested in the smart contract
functionality it offers. We also carry out a brief analysis of issues that are
introduced by negligent design of smart contracts. In the case of Namecoin, our
focus is how the name registration is used and has developed over time. For
Peercoin, we are interested in the use of proof-of-stake, as this consensus
algorithm is poorly understood yet used to move considerable value. Finally, we
relate the above to the fundamental characteristics of the underlying
peer-to-peer networks. We present a crawler for Ethereum and give statistics on
the network size. For Peercoin and Namecoin, we identify the relatively small
size of the networks and the weak bootstrapping process.
