This is an online and easy to use corpus for Chinese. Choose between modern and classical Chinese (the link goes to modern Chinese, as I assume that that is what most people are after). Enter your word and you'll be shown occurrences in the corpus. There are also some tools to analyse character and word frequency in a pasted text (字词频率统计) and a word segmentation and classification tool (汉语分词和词性自动标注), although the latter doesn't work on my system. You can also download spreadsheets with the search results, but you need to create an account to do so (which is free). The corpus only contains simplified characters.