German Codes

Germany used uncounted codenames in the second world war. Perhaps the most famous of these names are "Fall Weiß" (Case White), the campaign against Poland and "Seelöwe" (Sealion), the planned invasion of England.
 
The following table gives a small overview over the most important Codenames. The list will be extended at need or when information becomes available.

Operational Codenames

Fall Rot (Case Red)
In 1935 the Fall Rot was a study to defend against a surprise attack by France while defending the borders against Czechoslovakia and Poland. The 37 version of Fall Rot included offensive operations against Czechoslovakia with the aim of preventing a prolonged two-front war. In 1940 it was the second part of the western campaign - after the destruction of the BEF and the northern army of France it was, together with Fall Braun, the attack on the rest of the French army which was still entrenched in the Maginot line.

Fall Blau (Case Blue)
In 1938 a study from the Luftwaffe about aerial warfare against England. This would later become the "Planstudie 1939", a concept for the whole of aerial warfare.
In 1942 "Fall Blau" was the codename for the operations of Army Group South with the operational targets: Woronesh, Stalingrad, and Baku.

Fall Weiß (Case White)
The campaign against Poland.

Fall Grün (Case Green)
In 1937 the plan to attack and occupy Czechoslovakia. Executed without resistance in May 1938, after the Munich conference. In 1940 the plan for a frontal attack on the Maginot line. Later renamed Fall Braun.

Fall Gelb (Case Yellow)
The western campaign of 1940. Included the attack on Luxembourg, the Netherlands and Belgium. Originally planned as a frontal attack in coordination with Fall Braun, it was later modified into a armored attack through the Ardennes using a scheme developed by General von Manstein.

Fall Braun (Case Brown)
Plan to attack with Army Group C on the western front in June 1940 (see Fall Grün) to relieve Army Group A and B, if necessary.
Heeresgruppe C (von Leeb) was the army to execute this plan. Actually, in 1940 the Heeresgruppe C was far too weak to do anything but hold their position (some 19 divisions, most of them second class, for the whole southern front, including the Swiss border, against 37 French divisions and the Maginot line - some of their best among them). When Army Group C finally attacked for real, they were unexpectedly successful since the French no longer expected offensive action from them.

Weserübung
The occupation of Denmark and Norway, 9.4. - 10.6.1940.

Seelöwe (Sealion)
Planned operation for a 1940 naval invasion of England. Main landing area was the coast from Beachy Head to Dover, with side operations at Brighton and Exeter.

Adlertag (Eagles day)
Start of the planned air battle to win command of the skies.
Adlertag was August the 8th 1940. In an epic battle the British fighter-pilots managed to inflict heavy losses on the ill-equipped German bombers. Sealion was postponed, the battle of Britain would not happen - for the British, this battle was already won.

Artus-Plan / Kathleen
A planned invasion in Summer 40 of Ireland with the support of the IRA. Preparatory work on this plan was made by the IRA themselves.
See also the Irish timeline

Marita
The occupation of Greece.

Merkur
The air assault on Crete. This would only be executed in the case of an easy success of Marita.
The losses for the Germans in this operation exceeded those of the whole former Balkan-campaign.

Herkules
The planned invasion of Malta in 42. The operation was mainly canceled due to lack of oil for the Italian warships.

Barbarossa
Order 21 from December 18th 1940. The attack on Russia. Preparations would be concluded by May 15, 1941. The attack started on June 21, 1941.
The attack was postponed for a month due to several factors, the most important were the ongoing operations in the Balkans. The attack failed finally in the snowfields before Moscow.

Taifun
The battle for Moscow. The original goal was a kettle (envelopement) to destroy the army group Timoschenko, then a second, deeper, kettle with Moscow in it. The attack started on the 2.10.41.

Zitadelle (Citadel)
Attack on the Russian front at Kursk in 43.
This attack would become the largest tank battle in history. The attack was postponed several times by Hitler himself to allow the inclusion of new armor designs. This gave the Soviets the chance to prepare a stout defense and a counterattack. After this battle Germany never again gained the initiative.

Herbstnebel / Wacht am Rhein (Autumn Fog / Watch on the Rhine)
Codename for parts of the offensive in the Ardennes, 1944.
Part of the deception for this offensive was the secret operation "Heinrich", in which the foreign worker contingents of the Axis were infiltrated by German agents. They would disguise as members of the resistance movement and hire their fellows for anti-German activities. Fed with important (but wrong) information about the German military, the workers where given a chance to escape by their resistance-comrades. The information about the German troops acquired in this way was considered reliable by the Allied, enabling the Germans a last tactical surprise.

Kriegsmarine and Luftwaffe Codenames

Paukenschlag (Beat of the kettledrum)
A collective name for the first coordinated operations of German submarines on the US east coast.
Lack of a convoy system along the coasts of the United States made for a lot of fat targets for German submariners. Later operations had nice names like "Mordbrenner" or "Reißewolf". The diary of radio operator Wolfgang Hirschfeld gives a first hand account of the German side.

Himmelbett (Testerbed)
Since early 41 the central organization for the German nightfighters using the radars Freya and Würzburg.

Kammhuber-Linie
Operational zones for German nightfighters from Denmark to southern France.

Schräge Musik
The name for a "secret weapon" of the German nightfighters Several machineguns would shoot upward at an angle of 80°, to take advantage of the the weak underside of the British bombers. Originally this weapon was developed by private initiative.

Wilde Sau (Wild Sow)
A codename for the usage of single-engine fighters at night. These would fly without any radar, the pilot relying on eyesight.
Although successful in a few cases, the main problem for these fighters was to be over the right city at the correct time, something that never was really solved. Generally the idea was considered a failure.

Other Codenames

Alpenfestung (alp fortress)
A supposed mighty fortress in the German alps. The German variant of the Alpenfestung was actual inspired by rumors from the Allied side. Only minimal works were even started when the war ended.

Wolfsschanze (wolves entrechment)
The most used of many (at least 11) -Führer-HQs, near Rastenburg (East Prussia). The walls and ceiling of Hitler’s bunker had a strength of 7 meters.

Aufbau Ost (eastern build up)
1.8.40. The secret buildup of the infrastructure in the Generalgouvernement (the occupied part of Poland) to support Operation Barbarossa.

Blitzableiter
Study for the preparation of bacteriological warfare. 23.2.43

Mars
In August 43 some 40 soothsayers, fortune-tellers, astrologers and similar occupations were collected (from prison and KZs) on order from Himmler to divine the whereabouts of the captured Mussolini.

Reference

Most of the given information is from "Deutsche Decknamen des zweiten Weltkrieges", Werner Uhlich, Kurt Vowinckel Verlag, Berg am See (Germany) 1987, ISBN 3921655539.
Additional information comes from a multitude of other sources. See the resource-list for reference.