I tend to hear a lot of people mangling the pronunciation of German words around September (re: Oktoberfest time), and generally when trying to pronounce names and words from other Germanic languages. Somewhat ironic considering that English is a Germanic language too, but whatever. Anyway, here are a few words/names that I hear being mispronounced, along with a guide on how to pronounce it correctly.

For some reason, some English speakers have a tendency to ignore accent marks such as umlauts. Dön't! I mean, don't! Note that pronunciation guides are approximations. See the IPA spellings in brackets for the technically correct pronunciation.

Mjölnir (/ˈmjɒlnɪər/)
Aahhh, Thor's mighty hammer. This is pronounced as "mee'yurl-near," not "mee'yoll-nur." Be mindful that the "-ir" is pronounced as "ear," and not as "err"; similar to Mithrandir (/ˈmɪθrændɪər/; the Elvish name for Gandalf), which is pronounced as "Meeth-ran-dear."

Löwenbräu (/ˈlɜrvənbrɔɪ/)
A famous German brewery in Munich, and also the first part of the name of a famous Bavarian restaurant in Sydney (Löwenbräu Keller). Every year I hear people talking about the Löwenbräu Keller and referring to it as the "Low-wen-brow." <shudder> It's not. You say, "Ler-vern-broy."

Möbius (/ˈmɜrbiəs/)
I sometimes hear people say "MOH-bee-us," but it's "MER-bee-us." Ad infinitum.

Gestalt (/ɡəˈʃtalt/)
A word which has been widely used by Transformers fans since 1994 to describe Combiner team robots. I've heard it pronounced as "Guess-stollt" to "Jess-stollt." A more accurate approximation is "Guh-shtah'lt."

Ikea (ɪˈkeːˈa)
A lot of people say "Eye-kee'yah," but the correct pronunciation is "Ee-keh-ah." This is how it's pronounced in Japanese (イケア). Having said that, a lot of native Swedish speakers frequently mispronounce it as "Ee-kee-ah," but "Ee-keh-ah" is technically more correct because it's an acronym which stands for Ingvar Kamprad Elmtaryd Agunnaryd. The mispronunciation by Swedes is likely because Ikea is typically spelt without an accent mark on the "e." It used to be spelt with one (Möbel-IKÉA), but it's since lost the accent mark, and so there's been a vowel shift from é (eh) to e (ee), even among Swedish speakers.

Ægir/Ägir /ˈægɪər/
The Norse god of the sea. I frequently hear it being mispronounced as "Ey-Gar," but it's actually "Ag-gear."

Many words and names from Middle Earth are based on Germanic pronunciation, which is not surprising considering that Tolkien was a Germanic linguist. And so we have...

Smaug (/smaʊg/)
The "au" is pronounced as in the word "Audi," and not as in the word "naughty." So it's "SmOWg," not "SmORg." Things have gotten a lot better since the Hobbit films came out, but before then, I encountered a lot of people who'd pronounce it as "SmORg." I used to teach The Hobbit as an English text, and students were often reading it as "SmORg," no matter how many times I'd tell them at it's "SmOWg." It's also the same as the "au" as in "Sauron," which of course is pronounced, "SOW-ron," and not "SAW-ron."

Beorn (/bɛɔʀn/)
"BEH-orn," and not "BEE-orn." This is actually an Old English name (not all words and languages in Middle Earth were inventions of Tolkien -- the Rohirrim language is actually just Old English!).

Gandalf (/gɑn.dɑlf/)
The A's are pronounced as in the word "father," and not as in "apple." Thus you say, "Gun-dulf," not "Gan-dalf." Sir Ian McKellen actually pronounces it correctly as "Gun-dulf," and even made a point of it in one of the DVD special features interviews. It comes from the Old Norse word "Gandalv," which means magic wand or staff. Some people argue that it should be pronounced as "Gun-dulv," in keeping with its Norse origins, but Tolkien himself pronounced it as "Gun-dulf," thus making that pronunciation canonically official.