These functions are only available in the 1.4.0 beta (recorded here for personal reference if the main site disappears).

escape(@string);               // URL encodes a string
e(@string);                    // escape string content
%(@string, values...);         // formats a string

unit(@dimension, [@unit: ""]); // remove or change the unit of a dimension
color(@string);                // parses a string to a color
data-uri([mimetype,] url);       // * inlines a resource and falls back to url()

ceil(@number);                 // rounds up to an integer
floor(@number);                // rounds down to an integer
percentage(@number);           // converts to a %, e.g. 0.5 -> 50%
round(number, [places: 0]);    // rounds a number to a number of places
sqrt(number);                  // * calculates square root of a number
abs(number);                   // * absolute value of a number
sin(number);                   // * sine function
asin(number);                  // * arcsine - inverse of sine function
cos(number);                   // * cosine function
acos(number);                  // * arccosine - inverse of cosine function
tan(number);                   // * tangent function
atan(number);                  // * arctangent - inverse of tangent function
pi();                          // * returns pi
pow(@base, @exponent);     // * first argument raised to the power of the second argument
mod(number, number);       // * first argument modulus second argument

convert(number, units);    // * converts between number types
unit(number, units);       // *changes number units without converting it
color(string);             // converts string or escaped value into color

rgb(@r, @g, @b);                             // converts to a color
rgba(@r, @g, @b, @a);                        // converts to a color
argb(@color);                                // creates a #AARRGGBB
hsl(@hue, @saturation, @lightness);          // creates a color
hsla(@hue, @saturation, @lightness, @alpha); // creates a color
hsv(@hue, @saturation, @value);              // creates a color
hsva(@hue, @saturation, @value, @alpha);     // creates a color

hue(@color);           // returns the `hue` channel of @color in the HSL space
saturation(@color);    // returns the `saturation` channel of @color in the HSL space
lightness(@color);     // returns the 'lightness' channel of @color in the HSL space
hsvhue(@color);        // * returns the `hue` channel of @color in the HSV space
hsvsaturation(@color); // * returns the `saturation` channel of @color in the HSV space
hsvvalue(@color);      // * returns the 'value' channel of @color in the HSV space
red(@color);           // returns the 'red' channel of @color
green(@color);         // returns the 'green' channel of @color
blue(@color);          // returns the 'blue' channel of @color
alpha(@color);         // returns the 'alpha' channel of @color
luma(@color);          // returns the 'luma' value (perceptual brightness) of @color

saturate(@color, 10%);                  // return a color 10% points *more* saturated
desaturate(@color, 10%);                // return a color 10% points *less* saturated
lighten(@color, 10%);                   // return a color 10% points *lighter*
darken(@color, 10%);                    // return a color 10% points *darker*
fadein(@color, 10%);                    // return a color 10% points *less* transparent
fadeout(@color, 10%);                   // return a color 10% points *more* transparent
fade(@color, 50%);                      // return @color with 50% transparency
spin(@color, 10);                       // return a color with a 10 degree larger in hue
mix(@color1, @color2, [@weight: 50%]);  // return a mix of @color1 and @color2
greyscale(@color);                      // returns a grey, 100% desaturated color
contrast(@color1, [@darkcolor: black], [@lightcolor: white], [@threshold: 43%]);
                                        // return @darkcolor if @color1 is > 43% luma
                                        // otherwise return @lightcolor, see notes

multiply(@color1, @color2);
screen(@color1, @color2);
overlay(@color1, @color2);
softlight(@color1, @color2);
hardlight(@color1, @color2);
difference(@color1, @color2);
exclusion(@color1, @color2);
average(@color1, @color2);
negation(@color1, @color2);

iscolor(@colorOrAnything);              // returns true if passed a color, including keyword colors
isnumber(@numberOrAnything);            // returns true if a number of any unit
isstring(@stringOrAnything);            // returns true if it is passed a string
iskeyword(@keywordOrAnything);          // returns true if it is passed keyword
isurl(@urlOrAnything);                  // returns true if it is a string and a url
ispixel(@pixelOrAnything);              // returns true if it is a number and a px
ispercentage(@percentageOrAnything);    // returns true if it is a number and a %
isem(@emOrAnything);                    // returns true if it is a number and an em
isunit(@numberOrAnything, "rem");       // * returns if a parameter is a number and is in a particular unit

// * These functions are only available in the 1.4.0 beta