There are online miners that work in your browser (at least there were some, I came across several when I was doing micro earnings.) but these only utilize your CPU, and if you want to maximize your earnings you want to be using your GPU too.

There are many downloadable miners that you can use to utilize your GPU and CPU; you can find them on this forum, and there is also a section where you can find pools. (https://bitcointalk.org/index.php?board=14.0 & https://bitcointalk.org/index.php?board=41.0)

In addition many of these online miners and miners don't actually mine bitcoin - they mine another cryptocurrency like Monero or Ethereum, and pay you in Bitcoin. This is because the mining difficulty is too high for CPUs or GPUs, as there are ASIC miners that can outperform them easily.