Haskell libraries

Web

scotty
servant - DSL for describing, serving, querying, mocking, documenting web applications and more.
WAI - Interface between Haskell web frameworks and web servers.

DB

PostgreSQL ORM - Haskell ORM (Object Relational Mapping) and migrations DSL for PostgreSQL.
Squeal - Deep embedding of SQL in Haskell.

SDKs

Amazonka - Comprehensive Amazon Web Services SDK.

Other

generic-lens - Generically derive traversals, lenses, and prisms.
freer-simple - Friendly effect system for Haskell.
Capability - Extensional capabilities and deriving combinators.
Req - Easy-to-use, type-safe, expandable, high-level HTTP client library.
silica - Haskell optics with type errors useful enough to teach people.
aeson - Fast Haskell JSON library.
proto-lens - API for protocol buffers using modern Haskell language and library patterns.