Two dimensional painting applications can make interesting use of texturing and blending. An arbitrary image can be used as a paint brush, using blending to accumulate the contribution over time. The image (or paint brush) source can be geometry or a pixel image. A texture mapped quad under an orthographic projection can be used in the same way as a pixel image and often more efficiently (when texture mapping is hardware accelerated).
An interesting way to implement the painting process is to precompute the effect of painting the entire image with the brush and then use blending to selectively expose the painted area as the brush passes by the area. This can be implemented efficiently with texturing by using the fully painted image as a texture map, blending the image of it mapped on the brush with the current image stored in the color buffer. The brush is some simple geometric shape and the (s,t) texture coordinates track the (x,y) coordinates as they move across the image. The main advantage of this techniques is that elaborate paint/brush combinations can be efficiently computed across the entire image all at once rather than performing localized computations in the area covered by the brush.