Is there a way to design my own Photoshop filters?
A1:
There's an old plug-in called Filter Factory, which supposedly lets you do so. I've found it on the Web, but haven't really tried to do anything with it. It's a fairly complicated procedure, and the documentation is lacking. If you're really good at programming, Adobe makes specifications available for software developers who want to create plug-ins for Adobe products. Visit http://partners.adobe.com/public/developer/photoshop/sdk/topic_sdk.html for more information. You will need to supply your name and email address to access this site.
Do these weird filters, such as Fur and Squint and Water Drops, have any practical uses?
A2:
Just today, I saw a sign that featured a frosty bottle of Coca-Cola on a red background. The entire image was made to look wet with Alien Skin's Water Drops filter. Anything's practical if you have enough imagination to make it work.