There is a cool new string tension calculator browser app from D’Addario Strings and Planet Waves.

To get started, just answer the question if you wanna base your custom set off of exisiting D’Addario sets with NO and you can choose the following things step by step:

-your instrument
-number of strings (including 7, 8, and 9)
-individual string scale length (fanned frets, anyone?)
-desired string size or tension

Lots of things to play with. Get into it!