ManicTimer
Runs in the background and shows you (at any given moment) an overview of applications that were open that day, till that moment and how much time you spent inside these applications. It is overkill for what you want, but it is also not huge in size and, more importantly, a "set-'n-forget"-kind of solution. You can get your overview in the shape of a bar graph or numbers, whatever you prefer.
It comes in 3 flavors, the free one is the most limited, but that would be most ideal here.