With the add-on or plugin system, Miranda IM can implement the tabbed instant messages windows or tabbed IM chat windows, where all your chat sessions or instant messages windows with other buddies or friends can be grouped together as tabs within just one single window, just like tabbed browsing in IE7 and Firefox. By default, Miranda IM comes with the SRMM Unicode messaging module plugin (in Miranda IM, messaging module is implemented via addons rather than from core), which provides basic messaging functionality to send and receive instant messages, with one window opened per contact or chat partner.
To use tabbed instant messenger window or group multiple chats each as tabs in a window, several upgraded version or modification of messaging module SRMM has been developed as an addon to Miranda IM to provide the multiple tabs per window functionality.
TabSRMM is an advanced modification of the original SRMM message dialog plugin. It’s an advanced message dialog for IM and group chats with a tabbed interface and highly configurable feature, and has since grown to support many more features such as events notification and avatars. It also has a rich template system. If you just want the tabbed chatting window, probably this addon is too much of heavyweight to you.
Update: TabSRMM has become part of Miranda IM and also Miranda NG, and thus no download required.
Scriver is SRMM modification and can be used as SRMM replacement, which provides some new features like tabs or message grouping as well as better integration with IEView plugin. Scriver is just a little bit more advanced than the regular SRMM, with its configuration remains as simple as possible. Note that IEView is not necessary to run Scriver, because the plugin includes full support for RichEdit-based logs, which is used whenever IEView is absent or disabled. However, Scriver has much less features when compared with TabSRMM.
Update: Available for Miranda NG.