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Update: Pixoh recently became "Snipshot".
Although I've moved most of these cool little web apps over to Weblist, Pixoh is so brilliant that it deserves a mention here too. So what does it do? Well, Pixoh is an astoundingly responsive ajax-powered image editor - it's like PXN8, but more intuitive. Pixoh allows you to import pictures from Flickr or any website using a bookmarklet. It also allows you to export and save images in a range of formats (JPG, PNG, PDF, TIF, PSD, and BMP). Not to mention the ultra responsive crop, rotate and resize features. It's too early to look for a business model, but paid accounts would definitely work here.