While looking for plugins to implement ‘Private message system’ between users in wordpress, i found a not so-famous but awesome plugin called ‘Cartpauj PM’. The plugin has a very neat code, interface and functionality. The code is based on OOP with nicely defined functions.
You can cal any function anywhere to have a part of functionality of the plugin. The plugin also has a skin/mod available, which has some css, images and nice layouts for plugin’s frontend pages. The cartpauj website has a number of other plugins as well and also a support forum for their plugins.