Work out change

Mathematica: 110 chars

Sort[IntegerPartitions[Rationalize@#,Infinity,{10,5,2,1,1/2,1/4,1/10,5/100,1/100}],
    Length@#1<Length@#2&][[1]]&  

Usage

%[0.98]  
{1/100, 1/100, 1/100, 1/10, 1/10, 1/4, 1/2}  

Or

Tally@Sort[IntegerPartitions[Rationalize@#,Infinity,
                             {10,5,2,1,1/2,1/4,1/10,5/100,1/100}],
     Length@#1<Length@#2&][[1]]&  

(6 chars more) gives

{{1/100, 3}, {1/10, 2}, {1/4, 1}, {1/2, 1}}

For other denominations, just change the rationals table {10,....,5/100,1/100}

Windows PowerShell, 108 111 117

Very first attempt, ungolfed so far:

$i=+("$input"-replace'[^\d.]')
$args|%{0d+$_}|sort -des|%{$a=[math]::floor($i/$_)
if($a){$i-=$a*$_
"$a×$_"}}

Implementation notes:

1. Accepts the quantity to return via the pipeline
2. Accepts the list of currency denominations via the command-line
3. The quantity can be given with a currency sign; that will be stripped (in fact, anything non-numeric).
4. The list of denominations does not need to be sorted.
5. The program will output the largest change smaller than the requested quantity achievable with the given denominations, i.e. 1.5 for 1.53 if the 1-cent coin is missing.

If 3 and 4 do not need to be satisfied (i.e. I control the input format ;-)), then the following program suffices (71):

$i=+"$input"
$args|%{$a=[math]::floor($i/$_)
if($a){$i-=$a*$_
"$a×$_"}}

D: 225 Characters

import std.algorithm,std.conv,std.stdio;void main(string[]args){auto m=args[1].findSplit(".");void p(T,S)(T t,T u,S s){foreach(v;[u,10,5,1]){writefln("%s %s%s",t/v,v,s);t-=(t/v)*v;}}p(to!int(m[0]),20,"");p(to!int(m[2]),25,"/100");}

More Legibly:

import std.algorithm,std.conv,std.stdio;

void main(string[] a)
{
    auto m = a[1].findSplit(".");

    void p(T, S)(T t, T u, S s)
    {
        foreach(v; [u, 10, 5, 1])
        {
            writefln("%s %s%s", t / v, v, s);
            t -= (t / v) * v;
        }
    }

    p(to!int(m[0]), 20, "");
    p(to!int(m[2]), 25, "/100");
}

Only handles US currency. Takes the value as a floating point value on the command line (must have the leading 0 for values under 1 dollar). Does not accept $ as part of value. Outputs the number of each type of bill/coin on a separate line. E.g. an input of 1.53 results in:

0 20
0 10
0 5
1 1
2 25/100
0 10/100
0 5/100
3 1/100