One brake on the GIMP's popularity is that, while it boasts dozens of filters, a rival like Photoshop boasts thousands. You may only occasionally need a special effect that imitates a pencil sketch or a famous style of painting such as Impressionism or Cubism, but, when you do, having a filter to create the effect instantly saves serious amount of time. To help bridge this divide, the GIMP is reviving the User Filter from its 1.x releases. This filter is a kind of meta-plugin that allows users to import and manage Photoship filters or, if they have the knowledge, to write their own.
The GIMP User Filter is available from the project's SourceForge.net site as source code or as a Debian package that may or may not work on Ubuntu, to judge from mailing list chatter. Once you install it, you will find it under Filter - > Generic -> User Filter.
You can find links to Photoshop filters through the portal site Photoshop Filters.com. The most useful links on the site are Filter Factory, the largest filter repository for Photoshop, and The Filters, a list of other filter collections. Before you start User Filter, you'll want to download some of these filters, unzipping them if necessary. Since these filters are not written by the free software community, they are often not licensed, but, since they are available for download, you can generally assume that they are public domain or shareware, particularly if you are using them privately. Professional designers, however, might want to check for licenses more carefully.
Working with User Filter
If you have never worked with GIMP or Photoshop filters, your first reaction to the User Filter window may be pure panic. The Filter Interface and Filter Formula sections of the Filter Editor tab, as well as the entire Filter Control tab, may seem like gibberish in which the only sense you can see is that the number 255 refers to the top value in RGB color values.
Fortunately, if your only interest is in importing filters, you can ignore the controls for writing filters. To import a Photoshop filter, all you need to do is click the Open button on the Filter Editor tab and navigate to your download directory. By default, the file manager displays only .GUF (native GIMP files), so you need to change it to .8bf (Photoshop filters) or, better yet, since some of the downloaded filters may not have case-sensitive names and display their extensions as all-caps, to All files. When you have selected a filter, you can see a preview in the top left of the screen. To apply it to the currently active file in the GIMP, click the OK button.
Should you find the filter worth keeping, go to the Filter Directories tab to designate a storage directory for imports. If you save the filter with a .GUF extension, you can save time the next time you use the filter by going directly to the Filter Manager tab, which lists only imported filters.
So far, so simple. However, taking User Filter to the next level is probably a giant leap for most users. Using a duplicate copy of a filter, you can experiment with some of the controls, especially ones that change RGB color values, but you are unlikely to go farther without detailed understanding of how a filter is constructed.
Since User Filter does not come with help yet, you will need to find other sources to learn from. One starting point should be the Filters section of the online GIMP help site, followed the Scripting section, which may help you understand what you are dealing with. For more detailed information, try the Filter Factory Programming Guide and the GIMP Talk forum for plugins, filters, and scripts, and to ask specific questions. However, you will probably need to be more highly motivated than most people to master writing your own filter, since User Filter clearly assumes that you are already familiar with the process.
For most of us, User Filter's main utility will probably remain as a filter importer. Not all Photoshop filters import cleanly -- only about 80%, to judge from my more or less random experiments. Still, even that is a huge improvement in the number of filters compared to what comes bundled with the GIMP. It's enough, anyway, to help you shake the suspicion that, by not actually writing your own filters, you're doing the graphics editor equivalent of using a professional publishing package to compose your grocery list.