When launched, phpBB IM opens a new browser window that serves as the Main IM Client. This window automatically refreshes on a regular time interval (the length of which is configurable) and checks for new instant messages on each refresh. The Main IM Client also lists the current number of new private messages (with a link to phpBB's private message section), the users and numbers of registered users, hidden users, and guest users online. The list of online users includes a link to each user's forum profile and a "Message" link, where you can send the user an Instant Message. There is also a "Controls" section, which contains links that may be of use, such as Close Windows (closes all open message windows), Logout (does the same as Close Windows, plus logs you out of the forum and closes the Main IM Client), Preferences, and Check for IMs.
A smaller new window is opened for each new instant message, and you can reply or close the new windows while choosing whether or not to save the new message. Instant Messages themselves have full support for all features that private messages have, such as BBCode, HTML, Smilies and signatures. When sending a message, you also have the option of saving a copy in your private messages. Messages that aren't saved when you read them are automatically deleted on the next refresh of the Main IM Client.
phpBB Instant Messenger can be downloaded from the DarkMODs phpBB IM Summary Page. You can also find a demo, the README file, preview screenshots, and a support forum there.
Any suggestions for new features are very welcome.