PopOops blocks pop-up windows in Internet Explorer and runs in the system tray. It's user-friendly and has optional sound and balloon tooltips. Only compatible with Windows 2K/Me/XP.

One of the standout features of PopOops is how small and easy to use it is. Once you've installed the program, it will run quietly in the system tray, doing its job in the background without being too intrusive. Additionally, users have the option to enable sound and / or balloon tool tip notifications (only available for Windows 2K / Me / XP) to let them know when a pop-up has been blocked.
Another handy feature of PopOops is that it will automatically close any script error message boxes that pop up as a result of the blocking process. This means that you won't have to deal with any additional disruptions to your work or browsing experience.
Lastly, it's worth mentioning that PopOops is OSI Certified Open Source Software and is distributed under GNU General Public License (GPL). This means that it's freely available for anyone to use and modify, which could be a great fit for anyone who values customizability and transparency when it comes to their software choices.
Overall, if you're on the hunt for an effective pop-up blocker that's easy to use and customize, PopOops is definitely worth a closer look.
Version 2.2.1: XP styles support, friendly setup