Charlie Cook released the lastest Partisan Voting Index (PVI) based on results from last fall's elections. The index is an attempt to find an objective measurement of each congressional district that allows comparisons between states and districts.
For instance, this chart ranks each congressional district from the most Republican (Spencer Bachus's AL-06 at R+29) to the most Democratic (Jose Serrano's NY-16 at D+41).
Key finding from the re-crunched data: There are far fewer swing districts today than there were in the 1990s and Democrats now hold most of them -- 34 of the 50 most potentially competitive districts (those that fall in the R+2 to D+2 category).