Watchr is a small application that periodically checks an RSS feed for new images, and pops them up briefly in a corner of your screen. It was designed for use with Flickr, and may not work with anything else. If you have Java 1.4 or higher, you can try it now: Launch Watchr
When Watchr starts up for the first time, Java Web Start will advise you not to trust me. For the rest of these instructions, I will assume you have recklessly disregarded Java Web Start's sage advice.
After you ignore this dire warning, the next thing to come up will
be a feed button, like this:

Go to a Flickr photostream page - like
my photos, or
photos tagged "nyc",
or your contacts (if
you're a member). There should be a link near the bottom of the page
labeled "RSS 2.0". Drag that link from your browser onto the Watchr
button.
(Important: Don't drag the page URL itself onto Watchr. It won't work. Drag the RSS link.)
Every half hour, Watchr will check for new images on that photostream,
and pop them up for a few seconds each, like
this one:

If you click on the pop-up, it will open the photo's Flickr page in
a browser window.
If you miss the pop-up, you can always click on the feed button to see the photostream page, and all the images that have been added to it recently.
If the feed button is minimized, Watchr will continue to check for new images and pop them up. If you close the feed button, Watchr will stop checking for new images, but will remember the feed you were watching next time it starts up. To restart Watchr, you can use the Java Web Start control panel, or just come back to this page and launch it again.
Any time Watchr is running, you make it watch a different feed by dragging another RSS link onto the feed button.
Each time you launch Watchr (from the control panel or this page), it will check for a new version and automatically download it. This means you'll get the new features and the new bug fixes, but you'll also get the new bugs. If you want to tell me about the bugs, send email to eostrom@drowning.org. (That's me, Erik Ostrom. Hi!) I also accept email on other subjects.
Not sure if you have Java? Try launching Watchr first - maybe it'll just work! Otherwise, I recommend Java 1.4.2 (because it's what I'm using and I know Watchr works on it). For Windows or Linux or Solaris, you can get it, eventually, by going to Sun's download page and burrowing through their impenetrably designed web site for a while. (Try the "Download J2SE JRE" link. That's all I can give you. From there you're on your own.)
In addition to things like Swing and Web Start, I'm relying on Rome and JDOM to fetch and parse RSS feeds. (It should handle Atom feeds, too, I just haven't tried it.)
I plan to make the source available if anyone wants it; so far I'm just too lazy.
Ken Fox, who took the photograph shown above, wrote another program that does the same sort of thing, but with more features and in Python.
Here's my web site. It's mostly about music.