Thanks to Brancher, Chaos, Claire, Haruka, Jay-chan, Renfield, aaand... anyone I forgot ^_^
Ahou "Moron".
Akuma "Demon".
Akai "Red".
Aishiteru/Ai shite "I love you".
Anata An intimate form of "you"; see Kimi. Also a way of calling someone "dear" or "darling" (tends to imply that the addresser and addressee are in a very intimate relationship).
Anou... "Ummmm..."
Aoi "Blue" or "green"; also "pale" (as in, if somebody is looking pale and unwell).
Ara ara/oya oya "My my" or similar verbal punctuation.
Arigatto/Arigatto gozaimasu/Arigatto na All of these mean "thank you", but "arigatto gozaimasu" is the most polite.
Baka "Idiot" or "jerk".
Bara "Rose".
Betsuni "Nothing"; "it's nothing".
Bishounen Literally, "beautiful boy". Any male in anime who is unfairly gorgeous--for examples, see the Page of Pocket Bishounen.
Boku Male form of "I" or "me"; see also Ore and Watashi/Watakushi.
-Chan Affectionate suffix, mostly used by women--for example, A would address her close friend B as "B-chan"; a mother would call her daughter Sakura "Sakura-chan"... my pal Haruka informs me that if you introduce yourself as "[your name]-chan", you immediately brand yourself an airhead.
Chibi A style in which characters are depicted as being about five years old, although in a character's name "chibi" can also be the equivalent of the English word "mini".
Daijoubu "It's all right".
Domo Variously, "thank you" and "you're welcome"!
Doozo "Here". Usually said when offering somebody something.
Doozo yoroshiku (or just yoroshiku) "Pleased to meet you". "Yoroshiku" is less formal.
Doujinshi Sometimes (badly) translated as "fanzines", doujinshi are fan-created manga. There are always a metric pantload of them available on eBay (and I bought one myself! GO ME!)
Doushita Roughly, "what is it?" "Doushite", on the other hand, is "why". Be careful not to get these two confused. ^_^
Ecchi "Pervert".
Eh? "Huh?"
Fuku Literally, "outfit" or "suit of Western-style clothes"; can also refer to the uniforms worn by most Japanese high school students (mostly the girls), or the sailor suits worn by Sailor Moon and the other Senshi... Hey, quit laughing, guys...
Fugu Blowfish. The most extravagantly expensive dish in any Japanese restaurant... also a REALLY fun word to say. Try it... fugu, fugu, fugu! ^_^
Furigana The hiragana characters that appear next to kanji in books or manga aimed at younger (i.e., not middle-aged) readers.
Gaijin "Foreigner" (i.e., not Japanese).
Gambatte "Good luck"; "go for it".
Gochisousama deshita "Everything was delicious". Said after any meal--it's just a polite gesture.
Gomen/Gomen nasai An informal "I'm sorry" ("Gomen nasai" is slightly more formal).
Hai "Yes".
Hentai/hentai yaro Hentai literally means "pervert"; "hentai yaro" is a way of calling a guy a perverted bastard. ^_^
Hi "Fire" or "soul".
Hidoi Roughly, "mean".
Hiragana One of the three Japanese writing systems--hiragana is the phonetic alphabet used to write Japanese-origin words or names. It's generally the first one children learn.
Honki/Honto "Honki" is "serious"; "honto" is "seriously", "really", or "truthfully".
Iie "No". "Iie, iie" is the correct response to a compliment, by the way (this is like saying "I'm too modest to acknowledge that what you are saying is true").
Inu "Dog".
Itai! "Ouch!"
Itooshii "Darling".
Itta dakimasu Literally, "I receive"; the correct response when someone offers you something, or what you generally say before a meal (sort of like a really really short grace).
Ja mata/Ja ne "Goodbye", informally. Ja mata is more like "See you later".