Is there an alternative dictionary for OS X? Is there any better English dictionary software available to replace the simple default dictionary app in mac? 

I need some features like


one-click word lookup, 
hotkey to open the program, 
hotkey to lookup,
speech synthesis...
Cocoaspell is an open source dictionary. Cocoaspell is a MacOSX interface to Aspell.


  Aspell is an Open Source spell checker designed to eventually replace Ispell. Its main feature is that it does a much better job of coming up with possible suggestions than Ispell does. In fact recent tests shows that it even does better than Microsoft Word 97's spell checker or just about any other spell checker I have seen. It also has support for checking (La)TeX and HTML files, and run time support for other non English language.