Electric teakettle if you use an older style stovetop one. The lowest end ones I've seen are only like $15, and work pretty much as well as some nicer ones, just don't look as pretty, but they'll boil water faster than the stove will. Even if you don't make tea or similar, if you ever eat things like pasta that are cooked in boiling water, you can heat the water in the kettle first and then transfer to the pot to make things slightly faster, and use less gas if you happen to have a gas stove.
Not only are they useful for their typical purpose, but they boil water faster in pretty much any boil water use case. Use the water for your pasta, hard boiled eggs, etc.!
Haha, I am in the UK, so we all have at least 1. I have 2! One for my office desk because I am addicted to hot water bottles and nice tea. :) As a yankee transplant, I advocate an electric kettle hardcore to everyone in the US. Y'all don't know what you are missing.