In QCDPlayer, there is this feature, that you can click the system tray icon in different ways to manage the playback.
For example: A double right click=Next track; a single middle click=Play/Pause; Quad left click=Main menu
and so on.

It's very useful, and I don't think this plug-in exists for foobar2000.

Well, the infrastructure's there. I'm sure that someone suitably skilled at handling the systray could link it to the fb2k api in a very minimal timeframe.

You could even allow for multiple icons and configurable actions for each icon.

Well, that's just what I was thinking about this morning. Since I have a new notebook with no Multimedia keys anymore I need a new way of simple and quick change the playback. So I thought about an old plugin from winamp, which changed the trayicon something like this and every corner had another playback option.
Maybe I'll try and start creating a plugin, handling different clicks on the trayicon as a little beginning.

Or you could use global shortcut keys... 'Windows key' combinations aren't used much and are thus great for fb2k to steel.

Anyone remember STP (SysTrayPlayer)?

It was a audio player that sat in the systemtray and was controlled from its tray icon. The icon changed depending on the mode the player was in and reacted to clicks and , double clicks and holds. You could pause and play with a single click, move to the next or previous track with double clicks and control the volume by click and hold. I used it exclusively untill I moved to OGG (which it doesnt support) and someone recommended Foobar. Looking back at it now it feels very much like Foobar.

I love Foobar but really miss STP's trayicon GUI.

In my opinion if you want a spec for a trayicon based GUI you would have to try hard to beat STP.

Unfortunatley the developer left to join the army back in 02 and development stopped. Fortunately you can still find it if you look hard.

ftp://ftp.sac.sk/pub/sac/sound/stp.zip

I offer this as inspiration for a potential developer.

blammo, you're damn right. These tray features are incredible and very useful. Sure, as suggested using some global keystrokes can help, but already use global hotkeys by other programs and don't want to use keyboard, every second (e.g. when just browsing or relaxing, ..)

I also try to search for these tray APIs in Foobar but I didn't find them. Can any Dev maybe help me, finding them?

There are no such APIs in the foobar2000 SDK. See the description of the Shell_NotifyIcon Win32 Shell function on MSDN or the uShell_NotifyIconEx (iirc) UTF-8 wrapper function in utf8api.