Erlang: Riak https://github.com/basho/riak Riak is actually a layering of a few different projects including Riak KV, Yokozuna (Solr), Riak Core, etc. It was grown out of the Dynamo paper.

Haskell: Snap https://github.com/snapframework/snap Snap is another project built in layers (snap-server, io-streams, snaplets, snap-core). The 1.0 release makes some pretty massive structural changes behind the scenes changes with minimal breakage of the public api and io-streams is a very nice api to work with.

JavaScript: Underscore.js http://underscorejs.org/docs/underscore.html Underscore is a utility library that gives a nice overview of various techniques in JS, such as how to handle equality, use of apply, ternary operators, etc. Many functions have fallbacks to ECMAScript 5 native functions.