See also

Caution! There is no guarantee for the following information. They are obtained by guessing. In the future, the company may change them in an incompatible way.

System`

DuplicateFreeQ[list] (>= v9.0)

DuplicateFreeQ[{1, 2, 3}] // Print;  (* True*)
DuplicateFreeQ[{1, 2, 1}] // Print;  (* False *)

TensorQ[expr]
Optional arguments of Return[expr, func] and Break[func]
(http://mathematica.stackexchange.com/a/6757)

f[x_] := Module[{},
  Do[If[x > 5, Return[a]], {3}]; x
];
f[6] // Print;  (* 6 *)

f[x_] := Module[{},
  Do[If[x > 5, Return[a, Module]], {3}]; x
];
f[6] // Print;  (* a *)

System`Private`

InternalSeries[f, {x, x0, n}] (>= v5.1)

Func /: System`Private`InternalSeries[Func[x_], {x_, 0, n_Integer}] :=
  Sum[x^i, {i, 0, n}] + x^n * O[x];

Series[Func[x] / x, {x, 0, 2}] // Print;  (* 1/x + 1 + x + x^2 + O[x]^3 *)

SpecialFunctions`

Give the following a try:

?SpecialFunctions`*

HarmonicPolyLog[{m1, ..., mk}, x] (>= v9.0)
FunctionExpandHarmonicPolyLog[expr] (>= v9.0)
MultipleFiniteHarmonicSumS[{m1, ..., mk}, n] (>= v9.0)
MultiplePolyLog[{m1, ..., mk}, {z1, ..., zk}] (>= v9.0)
MultipleZetaValue[{m1, ..., mk}] (>= v9.0)
ShuffleProductExpand[expr] (>= v9.0)
StuffleProductExpand[expr] (>= v9.0)

It seems that the definitions of harmonic polylogarithms and multipolylogarithms are the same as Remiddi and Vermaseren and Vollinga and Weinzierl, respectively:

(* HPL 2.0, Mathematica 10.1.0 *)
<<HPL`;
HPL[{1, -2, 3, 0}, 0.7] // Print;
SpecialFunctions`HarmonicPolyLog[{1, -2, 3, 0}, 0.7] // Print;
SpecialFunctions`MultiplePolyLog[{1, -2, 3, 0}, {0.7, 1, 1, 1}] // Print;

-0.25112
-0.25112
-0.25112

Still the implementation is mysterious...

(* Mathematica 10.1.0 *)
Series[PolyLog[2, 1 - x], {x, 0, 1}] // Print;
Series[SpecialFunctions`HarmonicPolyLog[{2}, 1 - x] // FunctionExpand, {x, 0, 1}] // Print;
Series[SpecialFunctions`HarmonicPolyLog[{2}, 1 - x], {x, 0, 1}, Assumptions -> 0 < x < 1] // Print;

Pi^2/6 + (-1+Log[x]) x + O[x]^2
Pi^2/6 + (-1+Log[x]) x + O[x]^2
Pi^2/6 + (-1-2*I*Pi+Log[x]) x + O[x]^2

Less known functions

Only usage messages by ?Command are available.

ArgumentCountQ[head, len, min, max]
ArgumentCountQ[head, len, {m1, m2, ..., mi}] (>= v8.0)

TwoArgFunc[a___] := Null /; ArgumentCountQ[TwoArgFunc, Length[{a}], 2, 2] && False;

TwoArgFunc[1, 2] // Print;  (* TwoArgFunc[1, 2] *)
TwoArgFunc[1, 2, 3] // Print;  (* TwoArgFunc[1, 2, 3] with an error message "TwoArgFunc::argrx: TwoArgFunc called with 3 arguments; 2 arguments are expected." *)

ListQ[expr]
OptionQ[e]

In >= v6.0, one can use OptionsPattern and OptionValue for manipulating options.

StringQ[expr]