These are solutions that are just a Web Access layer that you can put over any existing mail server.
With its plugin framework, it is very easy to be expanded to your needs.
This is another webmail front-end wich should gain more and more popularity as the development is progressing. Currently, it is still under heavy development but the most recent version is already suited to be used with enough features and stability.
Like Squirrelmail, it use a strong plugin architecture that allows any developper to add or modify existing functionalities.
From a user experience point of view, we prefer the Roundcube interface which is more stylish when installed by default (with colors, icons, nice layout, ...) than the Squirrelmail one (where you need to install plugins if you want icons, colors, ...)
Unlike Squirrelmail, Roundcube doesn't have yet any calendar (agenda) functionalities which can be a big draw back, but for sites that just require pure webmail, it is a winner.