This geeklet will display the latest xkcd comic and the alt text that you normally get when you hold your mouse over the comic.
The alt.glet geeklet finds the image address for the latest comic from the RSS feed, downloads it to your Pictures folder and saves it as "latest-xkcd.png". You can change this name and location if you want by editing the last line of the command. From the RSS feed it then also gets the alt text and displays it.
The xkcd.glet geeklet just displays whatever image is at /Users/matt/Pictures/latest-xkcd.png. Unless your username is matt, you are going to have to change this to match your username.
You need to have both of the geeklet files running, even if you don't want the alt text. To hide the alt text, either delete the line echo $alt from the command, or just set the text opacity to 0%.
With a bit of tweaking this script should work pretty well for most webcomics. If anyone modifies it for another comic, it would be awesome if they could submit it to this site.
