In my first few weeks at work, I noticed that my colleague used ear muffs regularly at our open office workplace. I had recently realized that blocking external aural stimuli actually helped me focus better than listening to music. After discussing a while about this, I took the plunge and got the same one he had: Clear Armor Safety Ear Muffs. That was 4.5 years ago and I am happy to report that this has turned out to be an indispensible tool in my work kit.
The typical solution for noise isolation nowadays are active/passive noise isolation/cancelling headphones, which I use regularly too. But, I found that ear muffs are much simpler and worked better since I do not really need to listen to music these days.
There is a ton of information online about ear muffs, the varying levels of noise reduction they provide. The metric they seem to use for comparison in this field is Noise Reduction Rating (NRR). The pair I use is rated at 24 db for NRR and there are products which provide higher noise reduction. You could study those and decide which one to buy from among them. But essentially, these are extremely simple devices which just block out all the sound physically.
The Clear Armor pair was pretty tight and uncomfortable for many months in the beginning. But I can now wear them easily for an hour or two. Also, the stems of my glasses do introduce a gap in the muffs, which is unavoidable, so some external noise does creep in. At our home with noisy kids, I have taken to wearing these while reading my books and magazines (which I can do without my glasses). Without glasses, this does produce a tight seal and I can barely hear anything happening outside.
In office, wearing these large ear muffs also acted like a clear indication in our open office that I was working on something and should not be disturbed.
This is a pretty low-tech solution and I also like that aspect about it. Another added bonus is that such ear muffs are near in-destructible, no matter how you drop them, smash them or stretch them. This is another important factor in homes with brats like mine, who are always actively destroying expensive headphones.
From my experience, I can say that for working at the computer, to study, to read books, to focus, I highly recommend having a pair of ear muffs around. But they definitely take some time to break into. I do not wear them for multiple hours, but for shorts spans of concentration and focus: for writing/reviewing a design doc, reviewing an important PR, or studying a paper.