Alexander Heide has used MathMap to transform x-ray images of crystals.
Tom Rathborne has used MathMap to generate some very unusual Mandelbrot fractal images.
Laurent Despeyroux has a page with GIMP tutorials which make use of MathMap to create interesting effects.
The following are examples of MathMap expressions, together with their effect on two images. The left one is an image of Elisa Bridges (US Playmate of the Month December 1994), the right one is a grid with grid size 16. All these expression are included in the plug-in.

origVal(xy+xy:[sin(y/6),sin(x/6)]*3)

origVal(xy+xy:[5*sign(cos(y/6)),5*sign(cos(x/6))])

origVal(xy*xy:[cos(pi/2/Y*y),1])

origVal(ra+ra:[sin(r/3)*3,0])

origVal(ra+ra:[0,-(r/R-1)*pi/5])

origValRA(r,a+a%0.14-0.07)

origValRA(r*r/R,a)

gradient((gray(origVal(xy))+t)%1)
p=origValXY(x,y); rgbaColor(curve(red(p)),curve(green(p)),curve(blue(p)),alpha(p))
alpha = user_slider("alpha",0,6.28318530);
dir = xy:[cos(alpha),sin(alpha)];
ndir = xy:[-dir[1],dir[0]];
p = xy / m2x2:[dir[0],-ndir[0],
dir[1],-ndir[1]];
pt = dir * p[0];\nvec = xy - pt;
dist = -p[1] / R;
pos = 0.5 + p[0] / R / 2;
lower = 1 / (user_curve("lower",pos) * 4 - 2);
upper = 1 / (user_curve("upper",pos) * 4 - 2);
f = lower + ((dist + 1) / 2) * (upper - lower);
origVal(pt + ndir * f * R)

origVal(xy+xy:[rand(-3,3),rand(-3,3)])

p=origVal(xy); p=if inintv((a-(pi/20))%(pi/5),0,(pi/10)) then p else -p+1 end; if inintv(r%80,68,80) then p else -p+1 end

# Thanks to Herbert Poetzl
rd=0.9*min(X,Y);
if r>rd then
rgba:[0,0,0,1]
else
alpha=-(5/3)*pi; beta=(1/3)*pi; gamma=t*pi*2;
sa=sin(alpha);
sb=sin(beta);
ca=cos(alpha);
cb=cos(beta);
theta=a;
phi=acos(r/rd);
x0=cos(theta)*cos(phi);
y0=sin(theta)*cos(phi);
z0=sin(phi);
x1=ca*x0+sa*y0;
z1=-sa*-sb*x0+ca*-sb*y0+cb*z0;
if z1 >= 0 || 1 then
y1=cb*-sa*x0+cb*ca*y0+sb*z0
else
z1=z1-2*cb*z0;
y1=cb*-sa*x0+cb*ca*y0-sb*z0
end;
theta1=atan(-x1/y1)+(if y1>0 then pi/2 else 3*pi/2 end);
phi1=asin(z1);
origVal(xy:[((theta1*2+gamma)%(pi*2)-pi)/pi*X,-phi1/(pi/2)*Y])
end

origVal(xy)*rgba:[1,1,1,0]+rgba:[0,0,0,sin((r-a*6)/6+t*2*pi)*0.5+0.5]
abs(rgba:[sin(r/4)+sin(15*a),sin(r/3.5)+sin(17*a),sin(r/3)+sin(19*a),2])*0.5
grayColor(sin(x*y/180*pi)*0.5+0.5)
p=ri:(xy/xy:[X,X]*1.5-xy:[0.5,0]);
c=ri:[0,0];
iter=0;
while abs(c)<2 && iter<31
do
c=c*c+p;
iter=iter+1
end;
gradient(iter/32)