• watashi (私) / boku (僕) = me, I (boku used for males only)
• hai (はい) = yes
• iie (いいえ) = no
• nani/nan (何) = what
• namae (名前) = name
• sou (そう) = so that’s how it is, that’s how it is, is that so?, really, seemingly, etc.
• nai (ない) = that’s not how it is (literally means ‘not’, but when used in a single word sentence that’s usually what it means)
• ii (いい) = good (also used to reject an offer)
• dakara (だから) = therefore, so, etc.
• koko (ここ) = here
• kore (これ) = this (item near the speaker)
• sore (それ) = that (item close to the listener)
• are (あれ) = that (item distant from speaker and listener)
• hontoni (本当に) = really (as in “really?!”, but can also be used as in “He really needs to exercise.”)
• zettai (絶対) = absolutely, unconditionally
• yappari (矢っ張り) = also, as I thought, still, in spite of, absolutely, of course
• mochiron (勿論) = naturally, of course
• mattaku (全く) = good grief, honestly, seriously (expression of exasperation. As in “honestly, I don’t know why you keep doing that, it’s just stupid.”)
• hidoi (ひどい) = cruel, terrible, awful (usually means something like “you’re terrible” or “that’s awful”, etc.)
• matte (待って) = (please) wait (from the verb ‘to wait’, matsu (待つ) conjugated in the -te form)
• chotto (一寸) = hey! (usually used before matte, eg “chotto matte!”, usually used when the speaker wants the listener to wait)
  
• abunai (危ない) = dangerous (when shouted out, it could mean “hey, watch out!”)
• nee (ねえ) = Hey! (usually used to get people’s attention, not used to greet people. Note that it’s not pronounced as in “knee”, the extra e just elongates the first e, pronounced more like ‘neh’, except without the h. You get the idea.)
• hora (ほら) = look!, look at me!, look out! (Similar in function to “Hey!” in english, but different from nee in japanese)
  
• urusai (五月蝿い) = noisy, loud (Sometimes translates loosely to “shut up!” when used in a one-word sentence)
• anata (あなた) = you (Don’t use this if the person has told you their name already, it’s considered rude. If they have, just use their name in place of anata.)
• kimi (きみ) = you (familiar) (This is somewhat similar to ‘bro’ or ‘buddy’ here I guess, don’t use this in polite situations. I hear old people use this a lot, so I guess it could also mean ’sonny’, so I guess it depends on context. Males only really use this. Note that I’m not 100% certain on this one, if I’m wrong please point it out.)
  
• suki (好き) = pleasurable, fondness (There’s no Japanese verb for ‘like’, so use this. Sometimes pronounced ’ski’.)
• kirai (嫌い) = dislike, hate (There may be a verb for this, but I’m not entirely sure)
• desu (です) = not a real word, just used to end sentences when speaking in a polite manner. Can also be pronounced ‘des’.
• ano (あの) / eto (えと) / sono (その) = umm…, err…, well…, etc.
  
• doushitano (どうしたの) = what’s wrong? (usually pronounced doushtano)
• nandemonai (何でもない) = easy, harmless (often used as a response to doshitano)
• betsuni (別に) = nothing, not particularly (also a response to doshitano)
• douzo (どうぞ) = go ahead, take it, come in, etc. (this word has a lot of uses, can be used after someone knocks on a door, when someone is offering something to someone else, etc.)
• miru (見る)/mite (見て) = to look / look, respectively (mite can be used as an interjection, eg “Mite!” means “Look!” Mite itself is a request to look at something though.)
  

These are pretty much used all the time, some more then others, but I personally hear them used a lot in the things I watch. I find that the best way to memorize words is if you use them in context, even if it’s in the middle of an English sentence (eg, “I hontoni need to eat some food, dakara I’ll go to the kitchen.”), but, if possible, stick to Japanese sentences (unless you don’t know enough yet to use complete sentences). If you’re still having problems memorizing them, the next best way is probably to just watch stuff and listen for the words (although, this may not work with the words that are used less often).

Of course, if you want you can see a dictionary definition by copying the kana for the word and pasting it into a site like jisho.org.

(11/25/09) Update: added some more words that I forgot about

(11/26/09) Update: added some more words