What's the status of alternative full node implementations? In "Does running an alternative full node implementation benefit the Bitcoin network?", I asked about running an alternative full node implementation for the benefit of the Bitcoin network.

What is the current status of different alternative full node implementations with regard to


maturity (alpha/beta/stable)
performance
compatibility tests
code review
suitability for different purposes:


experimentation
merchants
as a wallet
as a lightweight wallet back-end
solely for the benefit of the network?



Update: Moved the list of implementations into an answer.
Update: Coin Dance has an excellent table of node implementations with descriptions and links at the bottom of their Bitcoin Nodes Summary page.



Below are the implementations I've found. I'm not sure if all of them qualify as full nodes.


btcd
(in Go, by Conformal Systems LLC)
Bits of Proof
(in Java, by bits of proof zrt.)
bitcoinj has a
fully verifying mode
(in Java, by Mike Hearn)
picocoin
(in C, by Jeff Garzik)
libbitcoin has an
example full node implementation,
also advertises to be a
completely independent and full Bitcoin stack
albeit only pre-alpha proof-of-concept with no GUI
(in C++, by Amir Taaki)
caesure
(in Python, by Sam Rushing)
Electrum-server,
uses bitcoind as a back-end
(in Python, by thomasv@bitcointalk)
Ufasoft Coin
(in C++, only for Windows, by Ufasoft)
Bitcore (in JavaScript. Note that it needs parts of Bitcoin Core to function)


(Sources: Core Development Update #4 by Gavin Andresen, Jun 21 2013 on the Bitcoin Foundation blog; bitcointalk.org forums)

Updated bitcoinj and libbitcoin status (based on information from
this bitcointalk thread);
Electrum backend
Added Bitcore