Hey Robin,
These words are usually called 語氣助詞. There are loads, but the common ones I see are:
嗎 (or 麼) - used to ask a question
他是你的朋友嗎?/他是你的朋友麼?
吧 - looking for affirmation, but you're quite sure already
他是你的朋友吧?
呢 - used for "what about...", or for changing the tone of a statement
他呢?
他是我的朋友呢
呀 - a form of exclamation, or as a way of softening a sentence
快來呀!
他是誰呀?
啊 - as an exclamation, as a reminder, to express doubt, to express hesitation, when listing examples (spoken only)
這篇文章寫得眞好
多加一件衣服啊
你打算怎麼辦啊?
我啊?還沒想好
他的人品啊、家世、學歷啊都是上上之選
啦 - generally a fusion of 了 and 啊
他早來啦
耶 - generally used only in Taiwan, softens tone
對耶
哇 - generally used to express "wow" or crying
哇!這件衣服眞漂亮!
哇哇大哭
哦/噢.喔 - generally used to express "oh" in realisation or understanding
哦,我懂了
哪/吶 - used to show surprise, usually in the phrase
天吶
的 - often used as a form of stress of to show something as already happened
他是要走的
這件事我問過他的
了 - in addition to being a grammar particle, this can also soften your tone and sometimes has no meaning
你怎麼在這兒了?
嘛 - to show something is obvious, to express a hope or persuasion, to express hesitation
中國人多嘛
留下來吃飯嘛
這個計劃嘛...
罷了、而已 - to express "that's it" or "nothing more"
好奇而已
我只是開個玩笑罷了,你又何必當眞
Hope that helps
Laurence