Well there's two "sites" to the UAL website, ual.com itself, and itn.net where all the reservations happen.
www.ual.com is running a Netscape webserver running on Sun Solaris (specifically Netscape-Enterprise/3.6 SP2 on Solaris )
www.itn.net is running an secure Red Hat version of the Apache webserver, using Sun Solaris hardware (specifically Apache/1.3.9 Red-Hat-Secure/3.1 mod_ssl/2.4.5
OpenSSL/0.9.4 (Unix) on Solaris)

It's kind of hard to differentiate what the application server or database behind the scenes at ITN is though, looking at the URLs. From the job descriptions I found at getthere.com (the domain owners of itn.net), it appears they're using a variety of technologies, including C++, Java, Corba, UNIX, Oracle, Tuxedo, etc. Basically all high-end, reliable, transaction processing technologies. Exactly what one would need for something like a large travel reservations system...