both of the folowing options would need to use cron
for an email sig, assuming the email uses file sig you would configure a cron job
(here is some documentation on how to configure cron
https://help.ubuntu.com/community/CronHowto)
to run
fortune >> sig
cron can be configured by default to do this daily, hourly, weekly, or monthly
you can also configure cron with webmin
http://www.webmin.com/ (a good web based tool for configuring various aspects of a linux box, useful for those who have limited experience manually editing config files)
note programs like webmin aren't a substitute for knowing how to manually edit the config files but a helper for common tasks nonetheless
forum sig? probably not unless you were hosting the forum locally on your box
well, theoretically possible but you would probably have to program something to authenticate yourself onto the forum, then send the proper POST values to the proper script based on the output of the command fortune and then be run by cron, possible but a lot of work especially for a novice programmer
cURL might be able to do it but i've never tried doing it and you'd still need either a php script or a shell script or a perl script to execute cURL with the proper options