I frequently watch "one thing at a time" sale pages such as Chainlove for good deals on (in my case) cycling gear. These sites sell only one thing at a time, at fairly steep discounts, until the item is gone, then they move to the next thing. This site provides software to let you know when new things come up for sale, but it's Windows-only. They also provide an RSS feed, which is slow enough that I get notifications about the latest sale just after it's over.
GeekTool/curl/perl to the rescue: The Chainlove site changes the title to the page each time they put something new up. This command uses curl to grab the chainlove.com page and send it to perl, which searches for the title tags and prints out whatever is between those tags:
curl -s www.chainlove.com | perl -nle 'print for m:<title>(.*)</title>:'
I then use GeekTool to run that command once every 10 minutes or so, and print the results to a "windowless window" on my desktop. There are probably better ways of doing this, but this works well. The same snippet of code can be modified to look at any desired webpage, or part of a webpage other than the title tag if so desired.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20081026190130972