The Kompressed Solar System Mod is a mod that takes the solar system, and compresses it down and rearranges it to add a new level of complexity to the stock game. It doesn't add any new planets, but it re-arranges the existing ones into a quite interesting configuration.
This mod re-arranges the solar system as thus:
Jool: orbits the Sun
-Eve: orbits closest to Jool
--Tylo: orbits Eve
---Dres: orbits closest to Tylo
---Eeloo: orbits farthest from Tylo
----Pol: orbits Eeloo
-Kerbin: orbits second closest to Jool
--Laythe: orbits Kerbin
---Vall: orbits closest to Laythe
---Mun: orbits farthest from Laythe
----Minmus: orbits Mun
-Duna: orbits farthest from Jool
--Moho: orbits Duna
---Bop: orbits closest to Moho
---Ike: orbits farthest from Moho
----Gilly: orbits Ike
For more information, check out the page on the forums here