Example 1: Here we used these settings: radius=3, flatten=95, dry=34. The big flatten setting gives a painted feeling, while the dry setting helps to not distort too much original picture.

Example 2: With a radius of 2.5, we respectively applied a flatten setting of 80 then -80. Notice that the yellow spot of the berry are better eliminated with a high flatten setting.

Example 3: Here a simple comparison between a flat median and a regular one.

Example 4: Here a flat median applied on the horizontal direction only.

Remark: Ive been experimenting a lot, about possible ways to estimate median without sorting datas. The plugin Flat Median represents the result I was able to reach.

Changelog:
version 1.1: New setting , dry. Basically, it gives more weight to the center pixel while computing the median-like value. This can help to get a more subtle effect.
version 1.2: This version prevent the aliasing that appears when increasing flatten setting to big values.
version 1.3: Allow to use less Ram.
version 1.4: Let the user choose if he wants to use less RAM, with the Lessen bit precision option.