smtpd is a lightweight mail server whose sole purpose is to receive incoming messages and deliver them to local users. Its main "feature" (or non-feature, really) is to be completely unable to relay messages to adjacent servers: if an incoming message is not addressed to a local mailbox, smtpd will reject it. Another convenient feature is its automatic grey-listing of clients that allows you to efficiently reject spammers based on their behaviour. It can handle incoming email traffic for an unlimited number of domains (i.e. act as a MX for these domains) without the need for any configuration (other than creation of users' mailboxes).
Streams is an I/O library designed to eventually replace the current I/O facilities based on using Handles. The main advantage is its strong modular design using typeclasses. It consists of small independent modules, each implementing one type of stream (file, memory buffer, pipe, etc.) or one part of common stream functionality (buffering, char encoding, locking, etc.). 3rd-party librarie can easily add new stream types and new common functionality. Other benefits of the new library include support for streams functioning in any monad, Hugs and GHC compatibility, high speed, and an easy migration path from the existing I/O library. It is heavily based on the HVIO module written by John Goerzen.