Resistors of Unusual Value
Mathematica, 151 122 characters

Expects the target resistance to be stored in r and the list of available resistors in l.

SortBy[Join[{#,#}&amp;/@l,Join@@(#@@@Union[Sort/@N@l~Tuples~{2}]&amp;/@{{"+",##,#+#2}&amp;,{"|",##,#*#2/(#+#2)}&amp;})],Abs[#[[-1]]/r-1]&amp;]


Less golf:

SortBy[Join[{#, #} &amp; /@ l,
  Join @@ (# @@@ 
       Union[Sort /@ N@l~Tuples~{2}] &amp; /@ {{"+", ##, # + #2} &amp;, {"|", ##, 
        #*#2/(# + #2)} &amp;})], Abs[#[[-1]]/r - 1] &amp;]


The output format differs from the suggested one but configurations are easily determinable. The output is a list of configurations. Each configuration is of one of the following forms:

{R1, Total}
{"+", R1, R2, Total}
{"|", R1, R2, Total}


So the first three elements of the output read

{{"|", 680., 2200., 519.444}, {"|", 1000., 1000., 500.}, {"+", 150., 330., 480.}, ...}


If you're fine with rational numbers, I could save two characters from omitting N@. That is, the first element (for instance) would be returned as 4675/9 instead of 519.444.