Gaim (now renamed to Pidgin, but the version I'm using isn't that new) comes with a "guifications" plugin to do "notifications", those little popup "toast" messages to tell you that someone's messaged you, someone's logged on or off, all that sort of thing. However, guifications is ugly and doesn't look like the rest of my desktop, and there is a proper notification thing for Linux, called libnotify. Some bright spark has written a Gaim plugin to use libnotify for notifications, but it's not available in Ubuntu edgy. It is, however, in the development release, Ubuntu feisty. Here's how to install it (and this acts as a very tiny HOWTO for how to install stuff from an Ubuntu release other than the one you're on).
Basically, this involves building the package from the source code. First, then, you have to tell Ubuntu that it can get source code from Ubuntu feisty. Go
System > Administration > Software Sources. In the
Third Party tab, click
Add, and put in the
APT line: textbox:
deb-src http://gb.archive.ubuntu.com/ubuntu/ feisty main universe
Add Source,
Close, and
Reload when it tells you that the information about available software is out of date.

Next, you need to get the Gaim libnotify package. I'm assuming here that you've built packages before; if not, you'll need to install
build-essential from Synaptic (or with
sudo aptitude install build-essential in a terminal).
Create a temporary folder to do the build in (
Places > Home Folder, then right-click in your home folder window and say
Create Folder, and call the new folder
tmp).
In a terminal (
Applications > Accessories > Terminal), change into your created temporary folder
cd tmp
and install first the things you need to build the package
sudo aptitude install libnotify-dev gaim-dev fakeroot cdbs
(you could also install those three packages from Synaptic if you prefer), and then fetch and automatically build the gaim libnotify package
fakeroot apt-get --build source gaim-libnotify
That will take a little while. Once it's finished, go back to your Home Folder window and look in the
tmp temporary folder you created. There should be a file called gaim-libnotify_0.12-1_i386.deb

Run that file. The Package Installer window will pop up; click
Install Package. This will install the new notifications (it may ask for your password to do the installation).
Now, you need to enable the plugin. If Gaim isn't running, run it (
Applications > Internet > Gaim Internet Messenger), and right-click on its notification-area icon; choose
Plugins:

In the Plugins window, tick
Libnotify popups. Remember to untick
Guifications if you were using it before! Close the Plugins window. I then had to quit Gaim (right-click the notification-area icon,
Quit) and restart it before the new notifications worked properly. But, lo and behold, nice pretty notifications when someone logs in!
Or sudo apt-get install gaim-libnotify then enable it in gaim plugins as their seems to be a precompiled package now (i think?)