Borderless Gaming is a decent app if you have a tendency to play PC games that don't natively support borderless window mode (slightly older games, for example). It's very simple to use, just one or two clicks and you're set.

That said, it does have its problems, whether or not it works depends on the game it is used on. I've found that games that run on Unreal Engine either have some problems when Borderless Gaming is toggled on the game (tested Borderlands, had graphical issues), or just refuse to listen to the app at all (again Borderlands sometimes, Viscera Cleanup Detail).

On the games it works on though, it is usually worth using if you alt-tab a lot or have multiple monitors.

Although the software is open source and available for free online, a small one-time payment isn't much if you want to show your support.