Mandarin Popup will display a tooltip containing the chinese character, pinyin pronunciation, and English definition of the word underneath the mouse pointer.
From the context menu, you can also choose to browse and search the underlying dictionary.
Dictionary Browser Notes:
Hit ctrl-c to copy the current highlighted entry to the the clipboard.
Hit "c" to copy the current highlighted entry to the search textbox.
Right click to bring up a context menu that allows you to decrease or increase the dictionary grid's font size.
When searching for pinyin, separate each word by a space. You can use tone marks or tone numbers. eg. "ni" will search for all entries with pinyin "ni" regardless of tone. "ni3" will only search for "nĭ"
ǚ can be entered with the tone mark or as u:3. eg. if you want to enter lǚ as pinyin, you can either enter "lǚ" or lu:3.
The developer can also be contacted on Twitter @mandarinpopup