Taking a quick look at nciku: I see that there are a ton of commercials, but that may not be a problem for your adult audiences.
But I love the ability to draw the character with your mouse and have the system generate a list of characters you might be looking for. It even looks like the system isn't "stroke-order" sensitive.
I learned Mandarin (with a Taiwanese accent and traditional characters) in an immersion environment, having showed up in Taiwan (to teach EFL) knowing only how to say "Hello," "Thank you" and "cat" in Chinese. Can you tell me more about you and your students' contexts?
I was bored with flash cards, dictionaries, and online subscription services so I made a couple of innovative apps for learning Mandarin Chinese both Traditional and Simplified, Pinyin and Bopomofo. Hanzi Warrior is a language adventure game (with all levels of HSK and TOCFL vocab) and Bopomo Blitz is a fast match-two based game for learning Zhuyin. Check out the videos on YouTube and here are some free codes. You can redeem these in iTunes for a free copy.