Manic Time is one such tool and the free version is completely without distraction. When you want you open it and get a colored timeline that you can hover over

Manic Time is one such tool and the free version is completely without distraction. When you want you open it and get a colored timeline that you can hover over to see what program was active and the title of the active window. I pay for professional to get a notification everytime I have been away from the keyboard for a while. Then I can manually tag that time and specify for myself if it is work related or not, and in case I work on multiple projects, which one it relates to. The paid version also gives me a summarized time sheet view of the week and can store screenshots if I want to (I use it, although if I need to use the screenshots for documentation, like when I had to configure live remote systems using Windows Notepad and with no access to VCS a few years ago, then I use something else that saves high resolution :-)