Pleco
This is the best mobile dictionary application you can get. Period. Pleco offers various famous dictionaries in digital format, has build-in Chinese handwriting recognition and has all kinds of extra features that are just too many to mention here. It includes a flashcard program that can be completely tuned to your preferences and support an SM2 like repetitive learning algorithm and a document reader that allows you to tap a word in a Chinese text to immediately lookup the dictionary definition.
If you are studying Chinese, you simply need to have this program. It's hard to estimate how many hours of work it saved me and how many new words I learned because it was easy to look them up on go. For me it was the main reason to buy a Windows Mobile device (it also runs on PalmOS and an iPhone version will be released soon) and being able to run Pleco will be my main consideration when buying a new one. Sure, $60 is a lot of money to many students, but believe me, it is worth every dollar you spend, you will not regret buying this! It's cheaper than most bulky electronic dictionaries that are often designed for Chinese students learning English instead of you. The Basic bundle costs $60, I myself use the $100 Professional bundle and might upgrade to the Complete bundle in the future.
Skritter
I was contacted in 2008 by the Skritter authors asking whether they could use my cards for a cool new project they were working on: creating a system to practise writing Chinese on a computer. The cool thing about this system is that you don't need a tablet, you can just use a touchpad or mouse to write the strokes and Skritter will transform them into the exact stroke if it's close enough.
Skritter also uses a repetitive algorithm to decide which word you need to practise next and will let you input the tones too. This service is absolutely great for both beginners and advanced learners! It's a paid service, but you can try it for free for two weeks. And if you are a ChinesePod Premium subscriber, you get a light version for free.
Warning: It is extremely addictive! Writing characters has never been so much fun. :-)
PlaySay
You don't have Pleco flashcards or some other fancy repetitive learning algorithm program on your smartphone? You feel these things are way to complicated? But you still want to be able to practise Chinese on the go without carrying around loads of paper flashcards?
Then PlaySay might be for you! The PlaySay image flashcards can be installed on your iPod or any other MP3 device or cellphone that can display images. You don't get any fancy learning algorithm, but it's great for cramming just before an exam and making use of those lost minutes waiting for the train to come. They are not free, but printing paper flashcards cost money too. You can sign up for a free demo with some basic flashcards.
I got the opportunity to test the HSK2 set and it worked quite well. I prefer a sophiscated learning algorithm myself, but I know many people for which this would be more suitable.
ChinesePod
Coming soon...