I thought of submitting this as a blog post or guide, but don't know if this would be more appropriate. Below are some useful websites that let you get your IP address; and check for privacy and security vulnerabilities like DNS leaks, whether your ad-blocker is actually working if you're using one, and connection speed and related stats like upload and download speed. You're strongly recommended to either bookmark these pages or create shortcuts to them for ease of access. This list is also intended to gather such sites in one place and may be updated to include even more useful URLs. Note, though, that I have visited these sites recently and just decided to share them with you instead of making an exhaustive list that contains every single website that offers the functionality mentioned. Please feel free to share any such useful websites in the comments. One more thing is that these websites can be visited on any device and platform, even though I posted this under the Windows category just because I couldn't find a more appropriate category to post about such cross-platform stuff not necessarily related to accessibility. The availability of these websites and the fact that they eliminate the need to install an app does apply to Windows as well as other operating systems like macOS, iOS and Android.
IP Checkers
Privacy and Security Tests
Leak Tests
Ad-Blocker Test
Note that you can just visit any website that does display ads to test whether your ad-blocker is functioning as intended.
Speed Test and Network Diagnostics
Other
Note that many of the websites listed above have overlapping use cases and functions.