SkypeTab is a tool that wraps Skype chat windows into a tabbed interface. Besides integrating the conversations, SkypeTab can also embed the Contact List into its wrapper.
Overall, SkypeTab is a really nice tool - if you don't move the window, because the Skype conversation windows lag a lot behind the tabs wrapper when moving the window and unfortunately the author says this cannot be fixed, at least not at this time. Nevertheless SkypeTab is a nice tool if you usually have a lot of Skype conversation windows opened and want to unclutter you desktop.
To use it, firstly run "SkypeTab" (from the Applications > Internet) and then run Skype.
Important: Skype Tabs only works with Compiz, Openbox and Xfwm.
Update: for SkypeTab Next Generation (which fixes the annoying window movement bug, works with other window managers too and comes with an Ubuntu PPA), see: Skype For Linux With Tabs Support Using SkypeTab NG (Next Generation).