I personally wouldn't use AppSnap because it analyses the installed programs on the local machine and saves this file to the %APPDATA% directory. Ketarin is good because you can add your own programs plus automatically download and extract with commandline switches, but very difficult to use and .NET app. Update-Notifier (If I remember) installs files on the local machine and has a limited number of monitored applications same goes for FileHippo updater!
Have a look at WebVersions http://www.portablefreeware.com/forums/viewtopic.php?f=4&t=5271
, because the problem I can see with http://www.changedetection.com/
is if a simple advert changes on the site you will receive erroneous emails! WebVersions analyses a number e.g. 1.0 and detects any changes to this number only. It takes the text before the number and after.
It doesn't support automatic downloading but you can enter any program (bonus!) have a look at the examples supplied.