For Ubuntu 11.10
The first thing you would need to do is take a .png icon file that you would like to use and copy it to the respective ~/.local/share/icons/hicolor/ directory. This will contain the following directories (which are the dimensions of the icons) : 16x16/apps/ 32x32/apps/ 48x48/apps/
I downloaded a free 16x16 free icon sampler and picked a 16x16 icon called Alien.png
I took the Alien.png file and copied it to ~/.local/share/icons/hicolor/16x16/apps, When you copy yours, you can do that through either the GUI or terminal.
From there you would need do the following in a terminal (you may need sudo to copy this over):
cp /usr/share/applications/pidgin.desktop ~/.local/share/applications/
Next edit the pidgin.desktop file:
vi ~/.local/share/applications/pidgin.desktop
Look for the line that says:
Icon=pidgin
This will need to change to the new icon name without the .png prefix...
Icon=Alien
Save your changes... Then logout and log back in.
Please let me know if this helps or if you need more assistance.
This is a problem in all JetBrain IDEs. In PyCharm (the one that I use) it's solved in the 2.0 version. It's likely solved in the corresponding IntelliJ version (likely 11, but I'm not sure).
You will need to go into the Tools Menu and select Create Desktop Entry. It will create a correct Desktop file in the correct place, so it will be added to your DE menu (including Unity Dash). Afterward you can initiate it from the menu and pin the icon or drag and drop it to the bar.
Note that the menu names are those in PyCharm as I don't use IntelliJ, but they are likely the same.
Best Answer
I just installed Unsettings and it's excellent, handles the launcher and panel colour, opacity etc. It does a lot of other stuff too. There's a short piece about it here:
http://www.noobslab.com/2012/04/install-unsettings-tweak-tool-on-ubuntu.html