Pleco is a godlike dictionary app. I wish there was a Japanese equivalent, just as good.
Pop-up dictionary, equivalent of rikaichan(there's also a Chrome version.
https://addons.mozilla.org/en-US/firefox/addon/zhong-wen/
You might look into anki.srs or other flashcard programs for learning the characters/vocabulary. If you use a popular book or study method the there is a great chance that someone already made a related anki deck but you can always make custom ones as well.
A friend of mine uses this website:
skritter.com
For interaction with native speakers:
There is always a relatively large presence of Chinese speakers (mind the time zones) on sharedtalk.com, especially the guys in the voice chat lobby starve for even mediocre English speakers.
On italki you can pay for professional tutors and teachers but there aren't a lot of people for language exchanges.
On lang8 you can let native speakers check out your texts and essays.
Or just join the Chinese equivalent of twitter/facebook right away.