HORDE
This is the most sophisticated package offering mail filtering into virtual mailboxes and PGP encryption. Some of the mail-reading options are a little difficult to locate, for example it seemed to me to be difficult to read HTML attachments (unless I was missing something obvious). The package has a View Source option (a little hard to find, though) which is extremely handy for the more technically-minded of us
as it not only allows you to view the full headers (i.e. the 'envelope') but it allows you to view the raw text of the message content.ROUND CUBE
A less sophisticated package, which does not offer filtering or PGP. It has a View Source option, which is easier to find than the one in Horde, and it allows you to download in EML format which amounts to the same thing.
SQUIRRELMAIL
A rather basic package, although I notice that in the last few weeks it appears to have magically upgraded itself to use a style sheet (or was that you, Cookie?). Its options for viewing and downloading attachments (including HTML text) are minimal and it has no filtering or PGP facilities. It has a view all headers option, but it does not allow you to view or download the raw text, or 'source'.