This is a simple app for Perl/Tk that can do DNS Lookups (IP address to hostname and vice versa) in X11. If you have the Net::DNS Perl Module (available from CPAN, in this directory) it can also do lookups of the MX records for a host.
A screenshot of it is shown on the left.
It can be downloaded here.
Comments, suggestions, bug reports etc. should be emailed to me at alan@whirlnet.demon.co.uk