On Linux, I use a .forward file of the form:

myusername
|/home/myusername/bin/wmail.pl
[download]

My wmail.pl file has a line:

@text=<>;
[download]

and then I process the contents in various ways (e.g., filtering some messages, putting some in a file I can read in a web browser, etc.) Pretty rudimentary, but it works well for me.
chas
(Update-remark: The .forward file sends my emails to the usual spool via the first line, and then I can do anything else with it with the script. The first line could be omitted; I included it to have a backup file.)