imgupscaler.ai is one of the most visible web upscalers, and for good reasons. You can use it without signing up, its pricing page listed 6 free images a day when we checked, and its site says results carry no watermark. That is more generous than our free tier, which gives 3 credits a day with a small watermark on results.
It is also broad. Besides upscaling to presets up to 16K, it offers a long list of tools, from old photo restoration and unblurring to photo-to-anime and a video upscaler. It accepts HEIC files straight from an iPhone, batches up to 20 images, and sells credit packs that never expire, which suits people who upscale in occasional bursts.
If you want one site for many different image jobs, or the most free images per day, ImgUpscaler is the better pick.