Johann George Elser, born in 1903, was a carpenter (and occasionally worked in metals and clock making) that had been living in Germany when the Nazis came to power. In 1939, Elser attempted to kill Adolf Hitler, having come to belief he must be removed from power for the Nazi party's restrictive attitute toward the lives and businesses of Germans and his intent to drive Germany into war over issues such as reclaiming the Sudetenland. Elser, having experienced World War I as a teenager, was alarmed at the prospect of a second world war. Unlike the other attempts to remove the infamous Fürher from power, Elser was not connected to any of the numerous high- (and not-so-high-) ranking members of the Wehrmacht (the German military from 1935 to 1945) and Abwehr (German military intelligence under the Nazis) that either were attempting to remove Hitler and the Nazis from power themselves or would at least support anyone who sucessfully pulled off a coup d'état.
Hitler made a 90 minute speech on 8 November at the Burgerbraukeller beer hall in Munich every year since coming to power in honour of 16 of his supporters who died in a failed attempt at removing the Weimar Republic (the German government after World War I but prior to Hitler becoming a dictator) from power in 1923. In 1938, Elser was present for this speech, taking note of when Hitler arrived, how long he spoke, where he spoke, when he left, etc. With this exact information, Elser had what he needed to plan how to kill Hitler the next year.
After the Sudetenland was ceeded to Germany in August, 1939, Elser worked meticulously to plant a time bomb within the podium of the beer hall without being detected. Over the course of several months, Elser would hide in the hall's gallery every night, waiting for the building to empty so he could begin his work. Using only a flashlight shrouded by a handkerchief, Elser slowly, carefully, and undetected chiseled a cavity bit by bit into a column near where Hitler would speak. Whenever a harder blow was needed to penetrate deeper into the column, Elser would wait for noise from the street outside to cover the louder sound he might make. This usually meant a lot of waiting for Elser as he worked in the pre-dawn hours of the morning. Elser told his landlord that he was working on an invention during the night at a shop open to him only after midnight (Elser didn't live in Munich and was staying there only to plant the bomb). Elser would work as much as he could during the night, leaving the beer hall sometime after it opened as though he were just another customer.
Elser used two fifteen day alarm clocks as timers for the bomb (which was made of 50 kilograms of explosives), which he fitted to a system of cogs and levers. When it was all set, Elser could have activated the timer up to exactly six days prior to when he wanted it to detonate the bomb. Once the bomb was in place, Elser put a thin steel sheet along the inside of the column so that if someone tapped on it, searching for anything hollow, the sound produced would not give away the presence of the cavity containing the bomb.
After placing the bomb and testing the timers a few times, Elser finally set them at 6:00am on 6 November, 1939 to go off 63 hours and 20 minutes later (9:20pm on 8 November, 1939). In the early morning of the eighth, Elser checked his bomb one last time (it was working perfectly) and then departed for Switzerland. Just after 8pm that night, Hitler entered the beer hall and began his speech, during which he spoke of the pain of British workers, the troubles of the major Western powers due to the Great Depression, and the economic success of Germany under the Nazis. The weather on that day meant that Hitler would have to travel back to Berlin by train rather than aircraft though, and Hitler needed to cut his usual 90 minute speech short. Hitler finished his speech and left the beer hall at 9:12pm... eight minutes before the bomb went off.
When the clock hit 9:20pm, the bomb detonated as it was set to, killing eight people, injuring over sixty others, and causing the ceiling of the beer hall to collapse. Elser never made it out of Germany: He was about 100 yards from Switzerland when he was arrested by German guards at the border. In Elser's pockets the guards found a clock spring, some cogs, a small aluminum detonator, and a postcard of the beer hall. When the guards heard of the explosion at the beer hall, Elser was handed over to the custody of the Gestapo who interrogated (and likely tortured) until he finally confessed to planting the bomb. Despite the Gestapo's interrogations, Elser continued to claim that he worked alone. There's been some speculation that Nazis Heinrich Himmler and Reinhard Heydrich were involved in Elser's plot, as at the time they would have gained much if Hitler had died (Himmler would have likely taken Hitler's place as Führer) but nothing has been found to support this idea.
Rather than receiving a swift execution, as most resistance members were by the Nazis (one member of the resistance, Count Peter Yorck von Wartenburg, was hanged only hours after his trial), Johann George Elser was kept as a special prisoner in Sachsenhausen until 1944, when he was moved to Dachau, still to be kept alive (possibly for a show trial after the war ended, assuming the Nazis were victorious). What he was being kept alive for was never revealed though, for on 9 April, 1945, Reichsführer-SS Himmler, Hitler's right hand man, secretly ordered Elser to be executed and blamed his death on a bombing raid by the Allies. This was, unfortunately, only a few weeks prior the Nazi surrender.
On 8 November, 1999, artist Wolfram Kastner spray painted in four different places in the city the phrase "I wanted through my act to prevent more bloodshed," (though in German, obviously) a quote attributed to Elser commenting on what he had done, and also spray painted Elser's name on a stone slab engraved with the names of Germans who resisted the Nazis (Elser's name wasn't engraved on it), in honour of what Elser had attempted to do.
Sources:
http://www.joric.com/Conspiracy/ResearchPaper.html#9
http://www.yad-vashem.org.il/about_holocaust/chronology/1939-1941/1939/chronology_1939_18.html
http://www.flamemag.dircon.co.uk/elser.htm