Most crypto-currencies are blockchain based. Essentially they use a distributed ledger to keep track of all transactions (ever) and come to a conclusion about the balance in any given wallet. The downside of this is the size of the ledger is enormous and keeps growing over time. Recently there have been some new coins that try to get away from this paradigm to provide something much more lightweight.
IOTA (MIOTA)
The foremost of these is IOTA. Currently sat at number 10 in the market cap rankings, with a value of $6 bln USD. That's actually significantly down both in terms of ranking and a peak market cap of over $14 bln. This coin promises to be the currency for future Internet Of Things devices. A low cost, low latency network run by millions (billions) of IOT devices all across the globe. It isn't a Blockchain per se, but uses some called a 'Tangle'. Essentially, transactions cascade across the nodes on the network - the idea being that once that cascade starts it is very difficult (but not impossible) for a bad actor to intercept and change.
IOTA has has mixed publicity recently due to a string of announcements (including one with Microsoft) which some would consider over-egged. Also, a Google suggests that a number of users have had problems with transactions going through. Binance, which trades IOTA, even suspending withdrawls for a time due to network congestion. That would seem odd as IOTA claim that the network should scale ('Scalable, Decentralized, Modular, No Fees').
I bought some IOTA and downloaded the software to get a node running. Given the lightweight nature of the protocol (all these IOT devices running as a node on the tangle), I was surprised to be offered the choice between a full and light client. Selecting a full node I was asked to provide the IP addresses of some neighbours. Reading the help on the IOTA page:
"In order to run IRI and sync with the network, you need to have neighbors which want to pair with you. The easiest and fastest way to get neighbors is through our Slack, where you can join the #nodesharing channel to find neighbors.
It should be noted that you have to share your own IP and port with your neighbors as well, because IOTA relies on mutual tethering for the data exchange. If you need help simply ask on Slack."
So hang on. My IOT cryptocurrency which is going to run on all these IOT devices by default requires each one to be set up with IP addresses of its neighbours? Maybe I've misunderstood something here, but that sounds like a pretty major problem. Googling the reason for this suggests that there are security issues associated with auto-discovered neighbours. I have to highlight that this is the first ever time I have tried a crypto currency and not been able to easily run a full node. If you've worked in internet connected products (and I have), the last thing you want to be doing is adding configuration settings to them, especially ones that might be affected by where they end up. This adds effort, labour and significant cost.
So what are the other options?
IoT Chain (ITC)
This appears to be a direct competitor to IOTA, based in China. It's currently 148 in the market cap rankings and isn't even a functioning crypto-currency yet. It's just ERC-20 tokens which can be traded in at some point. Looking at the timeline on their website, they are looking at a mainnet launch in Q1 2019. So, this is fine for a speculative purchase but it's hard to say anything more than that. I'm also wary of any currencies that are the x-of-China, given how hostile China has become to crypto lately.
Nano (XRB)
Nano, formerly RaiBlocks, has hit the news lately due to a loss of the coin from an exchange. However, so far the indications are that is the exchange's fault, rather than any fundamental weakness in the currency so it warrants a further look.
Nano takes a different approach to the problem. It gives each account their own 'Blockchain' to manage. These are further simplified by the fact they can be pruned to remove the history, leaving only the account balance. Transactions between chains are performed via PoS and what appear to be MasterNodes (representatives). PoW is used, but only as an anti-spam measure.
I'm still not entirely clear from reading the White Paper how the consensus across the network is formed. There is reference to a Ledger size so I assume that the Representatives maintain an overall ledger of balances of the individual child chains.
Also, the software is all still in closed beta. There is a web wallet (which seems to work well), but the Windows software is not yet available and the Linux software is basic. Running a node requires a command-line setup, so this is now the second currency where I haven't been able to run a full node. However, Google suggests that auto-peering is available in Nano. Certainly transferring Nano coins was quick and easy.
There is still a question mark in my mind with Nano, which is why would anyone run a full node if you can run a light client? Without fees on the network, there appears to be very little motivation. However, for now this is my IOT pick but I will keep an eye on 'Nelson' which is the proposed solution to the peering problem on IOTA while I await the full Nano wallet to trial.