Calculate practical numbers
Mathematica, 126 121 chars
Thanks to belisarius.
Using the formula on wikipedia.
f=(i=j=1;While[j&lt;#,If[And@@Thread[#[[;;,1]]&lt;2+Most@DivisorSum[FoldList[#Power@@#2&amp;,1,#],#&amp;]&amp;@FactorInteger@++i],j++]];i)&amp;

Examples:
f[1]


1

f[8]


18

f[250000]


2764000

It took 70s to compute f[250000] on my computer.