Excerpt (see article for full details on each one):

In this article, we list eight tweaks to make GIMP a more serious Photoshop replacement option. Version 2.6 was used to test the following tweaks, but past versions of the app should work as well.

1. Add the PSPI Plug-In
2. Install Layer Styles
3. Add CMYK Color Separation
4. Install Liquid Rescale
5. Add Essential Brushes
6. Anchor Windows
7. Install Animation Package
8. Enable GEGL