A good sig often starts or is embellished with a cloudy effect layer. This effect, which is easily attainable with Photoshop, is however a bit more of a challenge to create when it comes to us poor people who use the GIMP. This tutorial will explain in detail how to achieve this cool cloudy effect using the GIMP to create mean-ass sigs for forums or other.
On this article, I will discuss mainly how to create backgrounds, but with a bit of layer transparency, it can be put as masking layer too.
Once that you have selected the appropriate sig size, go to Filters->Render->Clouds->Solid Noise. A new dialog will now open, where you will then be asked to select various options related to your cloud.
-Random Seed: Determines the patterns of the cloud. Just click the new seed button until you get a pattern that you like.
-Detail: This determines the sharpness of you clouds. This is mostly personal preference. I like 1 and 8 to 15.
-Turbulent: This makes a darker cloud. I personally think it makes the sig too dark, but use what you like.
-X Size: How horizontally stretched your clouds are. I like anywhere from 1.8 to 4, but again, this is your choice.
-Y Size: Same thing as X Size, but vertically. I like anywhere from 1.5 to 3, personal preference yet again.
In this exercise, we will be using Random Seed 3589579885, Detail 4, X size 2.6 and Y size 2.1.
At this point, you will have something that looks like this:

OOooo, wow clouds! These are nice, but the color is a bit dull. To fix this, go to: Layer->Colors->Colorize. A prompt will now appear where you will select the Hue, Saturation and Light values. Experimentation is the best way to understand these values. For this example, I will be using Hue 222, Saturation 59 and Light 65. You should now obtain an image like this one:

Even more OOooo, wow! COLORED CLOUDS! Now lets add some signature stuff. On mine, I just cut and pasted a couple of pictures of the BMW logo, M3 logo and the M3 outline, but the sky is the limit when it comes to sigs. I also added a beveled border to mine. The result:

Yes, I know, the layout sucks big time. But anyways, you should now master the art of creating cloudy-style backgrounds.
All of my tutorials, plus other tech-related stuff can be found on my blog before it ends up her, so be sure to take a look once in a while.