Inbucket 
Inbucket is an email testing service; it will accept messages for any email address and make them available via web, REST and POP3. Once compiled, Inbucket does not have an external dependencies (HTTP, SMTP, POP3 and storage are all built in).
Read more at the Inbucket Website
Development Status
Inbucket is currently production quality: it is being used for real work.
Please see the Change Log and Issues List for more details.
Homebrew Tap
Inbucket has an OS X Homebrew tap available as jhillyerd/inbucket,
see the README.md there for installation instructions.
Building from Source
You will need a functioning Go installation for this to work.
Grab the Inbucket source code and compile the daemon:
go get -v github.com/jhillyerd/inbucket
Edit etc/inbucket.conf and tailor to your environment. It should work on most Unix and OS X machines as is. Launch the daemon:
$GOPATH/bin/inbucket $GOPATH/src/github.com/jhillyerd/inbucket/etc/inbucket.conf
By default the SMTP server will be listening on localhost port 2500 and the web interface will be available at localhost:9000.
The Inbucket website has a more complete guide to installing from source
About
Inbucket is written in Google Go
Inbucket is open source software released under the MIT License. The latest version can be found at https://github.com/jhillyerd/inbucket