Infinite scroll back in Empathy
The Nokia N9, N900 and other phones have popularized the idea of scrolling upwards to incrementally retrieve your chat history. Now you can have it on GNOME too, via Empathy. The code is now in Git master, so you can either grab it from there or wait till GNOME 3.8 is released in March this year.
Internally, it works using the TplLogWalker API that was recently added to telepathy-logger, and by adding a #prepend node to each of the Adium themes provided by Empathy. Currently it is not a part of the Adium specification. So if you are a vendor (eg., Ubuntu) shipping your own themes, then you should update them accordingly.
There is room for further improvements to the user experience, but it works and it can be very addictive once you get used to it.