The ultimate in sketching. Skitch offers users the ability to sktech online or use their available Mac app to do sketching offline. With over 3 million images shared, they have gained quite a market share. The web-based application allows you to upload and share images, organize those images, capture screenshots and write comments on them and share images easily on Facebook and Twitter. It has social networking elements as well. Users can comment on images and even comment with images. The offline app provides a few additional features. And you can upgrade to the "plus" version. Although they have the web-based version, it is kind of sad to see the offline applications are only available for Mac users. Come on, we Windows users need to do sketching and sharing of images as well.