Taskbar Shadow is a freeware portable app adds shadow effect to the Windows 7 taskbar at any position irrespective of whether it is on Top, bottom, left or right.  You can right-click on shadow on the taskbar and select Opacity and then you can drag the slider between 10 to 255 value to change its opacity.

taskabr shadow

Using Taskbar Shadow

Download and extract tool’s archive to a folder and run the Shadow app.  To exit, you can double-click or right-click on the Shadow and select “Exit” button on the app.

change opacity

Download Taskbar Shadow