I know that on the German highways (autobahns) there is no general speed limit, however parts of the autobahns have a posted speed limits (130 km/h or less). Are there any online maps showing the speed limits on the autobahns?
The problem with all these maps is that there is no official master-map showing the limits on a given section of motorway in Germany. Thus, it is either the map creator collecting the information by themself or one needs to rely on user-acquired content.
Furthermore, no maps can accurately account for roadworks; they will typically be on short notice and only there for a rather short time making it incredibly complicated including those on motorway maps. Also, large streches of German motorways have variable speed limits that may or may not be included in the entire map. (But since these variable speed limits are given from signalling bridges, it makes sense to have them included in one way or another.)
Stand: 2009. Eine Aktualisierung findet aus Zeitgründen nicht mehr statt.
(Status: 2009. No longer updated due to lack of time.)
Another map can be found at autobahn-speedhunter.com. Their data is primarily user-generated; however, the site’s copyright says 2012, so it may also be already outdated.
The map link provided is excellent and for the most part accurate. I have resided in Germany for many years and I am a daily Autobahn traveler (car and/or motorcycle). I disagree however that the majority of the Autobahn is limited to 120Kph. 130kph is still the recommended speed (when there is no limit) and the most posted speed limit... at least throughout RP, Hessen, BW and Bavaria.
There is a map of speed limits at http://product.itoworld.com/map/124 - not only for German highways, but for (theoretically) every road on the globe. The data seems to be taken from OpenStreetMap, so I suppose it is user-collected. I've just checked my hometown as an example: coverage there is quite good, but not complete, and the data mainly seems to be accurate, but some limits that were lifted last year are still in.