Sorry for reactivating this question, but I didn't find the right answer here.

In formatting numbers you can use 0 as a mandatory place and # as an optional place.

So:

// just two decimal places
String.Format("{0:0.##}", 123.4567);      // "123.46"
String.Format("{0:0.##}", 123.4);         // "123.4"
String.Format("{0:0.##}", 123.0);         // "123"

You can also combine 0 with #.

String.Format("{0:0.0#}", 123.4567)       // "123.46"
String.Format("{0:0.0#}", 123.4)          // "123.4"
String.Format("{0:0.0#}", 123.0)          // "123.0"

For this formating method is always used CurrentCulture. For some Cultures . will be changed to ,.

Answer to original question:

The simpliest solution comes from @Andrew (here). So I personally would use something like this:

var number = 123.46;
number.ToString(number % 1 == 0 ? "0" : "0.00");

You can use the decimal.ToString override to specify a formatting.

decimal amount = 120.00m;
string str = amount.ToString("0.00");

This can also be used when using String.Format.

Console.WriteLine("{0:0.00}", amount); 

In the case of your first rule, it cannot be done on one line.

decimal amount = 120.00m;
string str = amount.ToString("0.00").Replace(".00", String.Empty);

The format method automatically does the commas and periods or visa versa (depending on whether your profile is in the US or elsewhere)

Assuming the $ sign applies to all cases in your org you can simply use

Decimal input = 2000;
String output = '$' + String.valueOf(input.format());

Note the String.valueOf(). This is required because the input variable is defined as a Decimal.

Edit: I noticed a bug with the Decimal.format() method where it will not show any numbers past the decimal point if there are only zeros there, ex. $100.00. To solve this I came up with this method.

private String getCents(Decimal x){
    String y = String.valueOf(x);
    String z = '.';
    if(y.contains(',')) z = ',';
    y = y.substring(0, y.indexOf(z));
    if(x - Decimal.valueOf(y) == 0)
        return String.valueOf(x.format()) + z + '00';
    else return String.valueOf(x.format());
}

Then to update the example above, it would be:

Decimal input = 2000;
String output = '$' + getCents(input);

Yes you can do it with String.format:

String result = String.format("%.2f", 10.0 / 3.0);
// result:  "3.33"

result = String.format("%.3f", 2.5);
// result:  "2.500"

Use the str() builtin, which:

Returns a string containing a nicely printable representation of an object.

E.g:

>>> import decimal
>>> dec = decimal.Decimal('10.0')
>>> str(dec)
'10.0'