I recently heard about an excellent website that has awesome information on mountain passes all over Europe, plus many other mountain roads that are not passes: climbbybike.com. It is specifically geared towards cyclists, but others might also find the information useful. It has profiles of each climb that include the average gradient in each km, or each section of the climb, in a color-coded graph (see attachment). Plus the base height, summit height, length, average gradient, and a difficulty score. The difficulty score is based on a complex calculation that sums the difficulty of each section of the climb to get a total score, and it seems to give pretty indicative scores.

The attachment is the profile of the Umbrail pass, which goes from Graubunden (Santa Maria) to Italy (Bormio). It is the climb that I've done that has the highest difficulty rating (132.4), and is the highest pass in Switzerland (2501m). My wife and I did this at the end of a long tour across the Swiss Alps last summer, on fully-loaded touring bikes.