Angst: Fear. This has developed very powerful connotations from its use in Germanic philosophy to describe a soul-deep sense of fear about the self. However, it is also used somewhat satirically to describe the often-ridiculous emotional turmoil within teenagers, “teenage angst."
Ansatz: More commonly used in academic English than in colloquial, this word has been borrowed to describe the beginning or onset of an event or idea.
Blitzkrieg: History nuts will know this one well: literally “lightning war," in particular with reference to the rapid-fire strategy of the Germans during WWII as they marched steadily across Western Europe. In English, it is also used generally to refer to any fast strategy, either in business or in war.
Bratwurst: A staple of every “American" barbecue.
Delikatessen: Often slightly Anglicized to “Delicatessen," this refers to a shop that sells delicious, tasty things. Spy the “essen" in there, which means “to eat"?
Doppelgänger: Used to describe a look-alike of somebody, often in a sinister sense.
Ersatz: This adjective is used to refer to a mediocre imitation or substitution of the “real thing," whatever that may be.
Fest: Like a party, only a festival!
Firn: A mountaineering term to refer to snow from previous seasons, often exposed in the fierce heat of summer as last winter's snow melts away.
Frankfurter: An item of delicious food, which, like the bratwurst, is a staple of “American" barbecues.
Gestalt: This term has been borrowed particularly in the fields of psychology and philosophy to refer to a single whole concept created by a collection of individual concepts. It's a bit tricky to translate this into English; hence, its direct borrowing from German.
Glockenspiel: A musical instrument where metal bars are struck with a mallet to produce sound.
Hamburger: Another food popularly considered “American," even if does take its name directly from a German city.
Hinterland: A favorite word of journalists and poets to describe the backwoods and interior of a given country, usually with reference to its lack of development.
Kaputt: Broken! Usually spelled with a single t in English, “kaput."
Kindergarten: Literally “children garden," a place for children to start in on their education while still playing around.
Meister: A master of some skill. Often used comically in English.
Poltergeist: Any kind of noisy, disruptive ghost. An excellent pop culture example in English would be Peeves the Poltergeist from the Harry Potter books.
Pretzel: Think salty, wholesome and delicious.
Rucksack: Ever wondered what the “ruck" had to do with the “sack"? Turns out, it comes from the German word for back—so it is literally just a backpack. “Rucksack" in English is often used to describe a broader category of heavy-duty bags that do not go on the back.
Sauerkraut: Tasty or not? Regardless of personal feelings on the subject, “sauerkraut" has directly entered the English language to refer to yet another boiled cabbage dish.
Über-: This prefix and occasional preposition has a certain amount of gratuitous use online and in print to refer to an excessive amount of something, the prefix literally translating as “over." Über-Übering, anyone?
Übermensch: literally “superhuman" or sometimes translated “overman." The German philosopher Nietzsche developed this concept with some political results. In pop culture, it rarely has all that much to do with his actual thoughts.
Wanderlust: “Lust" in German has somewhat different connotations than in English: a simple pleasure in something, not necessarily tinged with anything sexual. Add this to “Wander," and the meaning becomes a desire to walk and travel and sometimes obsession.
Zeitgeist: Another ghost word, literally translating as “time ghost." This is used to describe the current cultural and political trends of any day and age.