There are lots of resources (web, app, social media, and even print) for increasing your vocabulary by one word per day. This post will cover ones on the web. Please leave a comment if you know of others.
For this approach to be most effective, I would recommend adding your daily words to flashcard lists (perhaps arranged thematically, or b y radical) so that you can review regularly.
Declan Software — Audio via Flash. Also makes iOS word-of-the-day apps for simplified and traditional Chinese. Funny that the simplified (left) and traditional (right) word for the day are not always the same.
Transparent language — Flash-based on the website, but you can get the word by e-mail, RSS, Facebook, Twitter, Yahoo, or iGoogle. Provides audio and an example sentence (on the website at least).