For me, a tool becomes bookmark-worthy when it consistently saves time. Speed and simplicity are important, but what really makes me return is trust. If I know a tool will work every time without ads getting in the way, forcing signups, or adding unnecessary steps, it earns a permanent place in my bookmarks.
I also like tools that help me discover something new rather than just complete a task. For example, I've bookmarked resources that let me explore random countries and learn interesting facts because they're both useful and engaging. The best tools solve a problem quickly while giving users a reason to come back later.
In short: reliability first, convenience second, and a little bit of curiosity-driven value makes a tool memorable. For example: https://randomcountrygenerator.info/