They are called Compressnow and Compressor.io and based on experience, I think they are the best web-based image compressors.
Like every other compressor API, Compressor.io will allow you to downsize any image without losing any quality. However, what makes Compressor.io stand out is its ability to handle JPEG, PNG, GIF and SVG files. As a matter of fact, all my GIFs on Misstechy must pass through Compressor.io before I upload. It reduces GIF images perfectly without losing any quality.
The second is Compressnow. Although only limited to JPEG images, its unique selling point is the ability to reduce your image to the barest minimum without affecting the quality.
To prove this, I compared the top web compressors online including Compressor.io
This is the original image with a file size of 46kb
TinyPic reduced it to 25.4kb
Compressor.io which is my favorite because of its ability to convert gif reduced it to 31.80kb
Finally, Compressornow reduced it to 22kb
Plus, you can actually set the compression level (The image above was set to 46) – this gives you the flexibility to find the optimal between small image size and image quality.
Now you see why I rate these two guys high?
Meanwhile, I am thinking of bringing back the image upload feature in the comment box, what say you?