Everywhere I've been I pick up a few things from the local language that enter my English vocabulary for good. In Boston it was mostly strange pronunciations that we thought were funny ("stahhhhvin" for starving), although I've been known to still let a "wicked" slip out here and there. There are several German words that I always use now, even in English!
kippen/kip - This is the word for what you do to the German window when you have it tilted open from the hinge at the bottom. I have no idea what the English word would be because I never saw windows like that until I came here, which is probably why we always use "kip" even when speaking English.
Mief - This is what's in a room that's been all closed up with a lot of people in it, and it's all stuffy and horrible. Dict.leo says the English word for this is "fug" but I never heard it before. I don't know if we use that word in the US. In any case, it doesn't sound as awesome as Mief anyway, so I think Mief is in our vocabs to stay.
Pfand - In English I guess this is usually called the return - the money that you pay extra for a can/bottle/etc. We deal with the concept a lot more often here because recycling is a bigger thing, the Pfands are much bigger than in the US, and there are Pfands on the mugs, glasses, plates etc. that you eat off of when you go to a festival. Hence, the German word for it has really sunk in, because we never talked about returns that much in the US.
Spargel - Who could call it asparagus after learning this awesome word?
Kalk - In English this is lime or limescale. I never had a problem with it like I have here, so I never really needed to call it anything in English. Kalk is a somehow more apt name anyway.
The following are words we never bother to translate to English when talking, but which I doubt would stick to our English if we left: Rathaus (town hall), Bahnhof (train station), Nebenkosten, Betriebsarzt.
PS. My German friend made up the word "somewhen" last night when trying to think of the word "sometime". I like it.
PPS. Damon was just flipping channels and came across a show with 7 guys playing music on buckets and stuff in a vineyard, singing about how great the wine queen is. SWEET.