There are a lot of different effects in Paint.NET that you may use for the benefit of your image (or not). At any time you may press Ctrl+F to repeat the last effect you applied, with the same settings.
Effects Menu
The effects menu is where all of the different effects will show. Some are packaged with Paint.NET; others must be downloaded and extracted into the Paint.NET\Effects\ folder. Only the default effects will be dealt with here.
Blurs
There are six default blurs.
Gaussian Blur
Gaussian Blur is just your average blurring. Mostly, it is used as a softener to remove overly sharp edges, or pixelation.
Median
This blur reduces noise. As of the 3.20 Release, it's now under 'Noise' instead of 'Blurs'.
Motion Blur
This blurs the image in a way that makes it look like it is moving in a certain direction.
Radial Blur
Blurs the image like it is spinning around the center of the image.
Unfocus
Makes it appear like you are seeing it, but not looking at it. Seeing is believing (Use it and you'll know what I mean).
Zoom Blur
Makes it appear that you are zooming past the image. Once again, seeing is believing.
Distort
There are four different default distortions.
Bulge
Bulges the image out at the center. Like a pot belly.
Polar Inversion
Um. Just use it. I don't think it's really useful.
Tile Reflection
Sort of cuts it into tiles and makes it look like it's reflecting off those tiles. Looks really cool, please try it.
Twist
Could be a KoRn song. Twists your image around the center. The maximum value is 100, which is NOT 100 degrees.
Effects Menu II
The effects after the popouts.
Add Noise
Creates a static effect. The intensity is the amount of static, and the saturation is how color-ish it is.
Clouds
Creates clouds using your primary and secondary colors. How simple and self-explanatory.
Edge Detect
Detects edges and colours the edges in various colours, the inside is then coloured black.
Emboss
The good old emboss effect. It... embosses stuff. Somewhat self-descriptive.
Frosted Glass
Have YOU ever looked through Frosted Glass? It makes stuff look like your looking at it through a layer of Frosted Glass.
Glow
Lets you have a glow on an image.
Oil Painting
Makes it look like an Oil Painting.
Outline
Takes every pixel in the whole image, then adds an outline onto it.
Pencil Sketch
Makes the whole image look like a Pencil Sketch. Very effective, but only black and white.
Pixelate
Expands the pixels in a similar fashion to the zoom tool in MSPaint.
Red Eye Removal
Only put this here to make sure you know it exists. It really needs no explaining.
Relief
Makes the image a relief of itself. Think of it as making your image 3D, then making it look really bad.
Sharpen
Think unblurring, but ineffective.