Can anyone give a recommendation for a simple X-based equivalent of mail / mailx for reading the mail I get from cron jobs?
Evolution is fine, but heavy. Sylpheed is simpler and doesn't use as many resources.
If you don't want Evolution nor Sylpheed, Thunderbird also allow to get /var/mail mail. You just have to create a new account and choose "movemail" as account type. See mozilla wiki for more info.
Evolution can be set up to read system mail. When setting up an account, simply choose "Local Delivery" as the server type on the "Receiving Mail" portion of the setup. Then, on that same page, set the "Path" to /var/mail/ or wherever your mail spool is.
The "Local Delivery" option will delete the mail from /var/mail/. If you use the "Standard Unix mbox spool file" will not.
Whether or not you want to call Evolution "simple" is another matter.
You could write a shell script that calls a command-line mail tool and then uses notify-send, (g)xmessage or zenity to display the individual messages.
exmh is a rather lightweight X-windows front end for mh, that will quite easily read your local mailbox.