Some of the more popular webmail installs (Horde, SquirrelMail) are a bit slow and not very good looking. Yesterday I installed RoundCube and was very impressed. It really behaves like an online application and looks wonderful. It’s responsive and quick with searches.
Roundcube is quite easy to install and runs on a standard PHP/MySQL configuration. Change a few lines in the code, create a database and you’re ready to go.