I find the ability to forward messages to www.30boxes.com and have them added to my calendar extremely useful.
I created an applescript to slightly automate the process:
tell application "Mail"
set theMessages to the selection
repeat with thisMessage in theMessages
display dialog "30 Boxes subject:" default answer (thisMessage's subject as string)
set theSubject to text returned of result
set newMessage to make new outgoing message
tell newMessage
set subject to theSubject
set content to thisMessage's content
make new to recipient with properties {address:"add@30boxes.com"}
end tell
send newMessage
delete newMessage
end repeat
end tell