I wish I had found a list like this a few years ago.  I've searched the web and compiled the following list for your enjoyment.  Many are probably considered basic with a few very cool, complex curves thrown in.  The list is divided into the coordinate systems that you will have to choose when creating the datum curve.

To initiate the command that will allow you to try these, go to Insert>Model Datum>Curve>From Equation. 

After the Equations section see the section title Links to find PlanetPTC discussions and videos that have demonstrated and, in some cases, explained the curve from equation command in more detail with ways to incorporate relations and parameters.

Attached is a Creo Elements/Pro 5.0 part file with all of the equations included.

In the comments, please share any equations or links that you know.  Other suggestions are welcome, too.

EQUATIONS

Cartesian Coordinates:  x, y, & z

The z variable is not necessary, but when used will give the curve that extra dimension. If in doubt, try z = t*10.

Sine

Cartesian coordinates

x = 50 * t

y = 10 * sin (t * 360)

Rhodonea

Cartesian coordinates

theta = t * 360 * 4

x = 25 + (10-6) * cos (theta) +10 * cos ((10/6-1) * theta)

y = 25 + (10-6) * sin (theta) - 6 * sin ((10/6-1) * theta)

Involute

Cartesian coordinates

r = 1

ang = 360 * t

s = 2 * pi * r * t

x0 = s * cos (ang)

y0 = s * sin (ang)

x = x0 + s * sin (ang)

y = y0-s * cos (ang)

Logarithmic

Cartesian coordinates

z = 0

x = 10 * t

y = log (10 * t +0.0001)

Double Arc Epicycloid

Cartesian coordinate

l = 2.5

b = 2.5

x = 3 * b * cos (t * 360) + l * cos (3 * t * 360)

Y = 3 * b * sin (t * 360) + l * sin (3 * t * 360)

Star Southbound

Cartesian coordinate

a = 5

x = a * (cos (t * 360)) ^ 3

y = a * (sin (t * 360)) ^ 3

Leaf

Cartesian coordinates

a = 10

x = 3 * a * t / (1 + (t ^ 3))

y = 3 * a * (t ^ 2) / (1 + (t ^ 3))

Helix

Cartesian coordinates

x = 4 * cos (t * (5 * 360))

y = 4 * sin (t * (5 * 360))

z = 10 * t

Parabolic

Cartesian coordinates

x = (4 * t)

y = (3 * t) + (5 * t ^ 2)

z = 0

Eliptical Helix

Cartesian coordinates

X = 4 * cos (t * 3 * 360)

y = 2 * sin (t * 3 * 360)

z = 5

Disc Spiral 1

Cartesian coordinates

/* Inner Diameter

d = 10

/* Pitch

p = 5

/* Revolutions

r = 5

/* Height; use 0 for a 2D curve

h = 0

x = ((d/2 + p * r * t) * cos ((r * t) * 360))

y = ((d / 2 + p * r * t) * sin ((r * t) * 360))

z = t * h

Butterfly

a=cos(t*360)

b=sin(t*360)

c=cos(4*t*360)

d=(sin((1/12)*t*360))^5

x=b*(exp(a)-2*c+d)

y=a*(exp(a)-2*c+d)

Fish

a = cos (t * 360)

b = sin (t * 360)

/* As "c" increases the fish gets fatter until it transforms into a figure 8.

c = 10

x = (C*a-20*((b)^2)/1.5)

y = c * a * b

Cappa

/* "c" is a scaling variable

c=20

/* Revolutions

r=1

/* Height

h=0

x=c*cos(t*r*360)*sin(t*r*360)

y=c*cos(t*r*360)

z=t*h

Star

/* "a" & "b" are scaling variables

a=2

b=2

/* If, r=2/3 ----> astroid

/* If, r=2 ----> ellipse; when a=b, its a circle

/* r cannot equal 1

r=2/3

x=a*(cos(t*360))^(2/r)

y=b*(sin(t*360))^(2/r)

z=0

Bicorn

/* "c" is a scaling variable.

c=5

a=cos(t*360)

b=sin(t*360)

x=c*a

y=c*(a^2)*(2+a)/(3+b^2)

Talbots

/* "c" is a scaling variable.

c=10

a=cos(t*360)

b=sin(t*360)

x=C*a*(1+exp(2)*(b^2))

y=C*b*(1+exp(2)*(b^2))

Cylindrical Coordinates:  r, theta, & z

Spiral

Cylindrical coordinates

r = t

theta = 10 + t * (20 * 360)

z = t * 3

Circle Spiral Column

Cylindrical coordinates

theta = t * 360

r = 10 +10 * sin (6 * theta)

z = 2 * sin (6 * theta)

Helical Wave

Cylindrical coordinates

r = 5

theta = t * 3600

z = (sin (3.5 * theta-90)) +24 * t

Basket

Cylindrical coordinates

r = 5 + 0.3 * sin (t * 180) + t

theta = t * 360 * 30

z = t * 5

Disc Spiral 2

Cylindrical coordinates

R = 50 + t * (120)

Theta = t * 360 * 5

Z = 0

Apple

Cylindrical coordinates

a = 10

r = a * (1 + cos (theta))

theta = t * 360

Spherical Coordinates:  rho, theta, & phi

Butterfly Ball

Spherical coordinates

rho = 8 * t

theta = 360 * t * 4

phi = -360 * t * 8

Spherical Helix

Spherical coordinates

rho = 4

theta = t * 180

phi = t * 360 * 20

UFO

Spherical coordinates

rho = 20 * t ^ 2

theta = 60 * log (30) * t

phi = 7200 * t

Unnamed

Spherical coordinates

rho = 200 * t

theta = 900 * t

phi = t * 90 * 10

LINKS

Peruse the links for more equations and explanations as to how they work.

Web Links

1. Involute Gears
2. Power Tools: Curves by Equation
   1. This gives details about using Pro/E dimension references in the equation to give it a parametric touch.

Links to curve-from-equation Discussions on PlanetPTC:

1. Curve from Equation Sample for Newbies
2. Capto
3. How to Create a Curve from the Equation? Does Anybody Know?
4. Datum Curve from Equation Driven by Parameters
5. Constant Force Spring Model
6. Involute Helical Gear Geometry

Links for related PlanetPTC content:

If you think curves from equations are cool, then the following is right up your alley.  These delve into the use of variable section sweep and the trajpar variable.

1. From E-learning with Vladimir Palffy
   1. http://communities.ptc.com/blogs/vpalffy/2011/02/09/user-defined-springs
   2. http://communities.ptc.com/blogs/vpalffy/2011/11/27/sweep-and-trajpar
2. Fun with Helical Sweeps and Trajpar
3. Conch Shell in Creo Parametric
4. How to Create Twisted Wires