If you use Google services such as Google Reader, Google Drive, YouTube, Contacts, Google+ Circles, Latitude, Picasa Web Albums and a few more, you can download all of your stuff to your computer.
A real useful web service for all the designer who want to know what colors have been used on a certain image. You can even download a Photoshop Swatch file containing the color palette of the image.
If you are getting a vast number of email notifications sent by your Twitter account and they started to annoy you, this is how you disable and manage them.
If you are tired and annoyed by the app requests and notifications your friends keep sending on Facebook, this is how you get rid of them without having to unfriend your friends.
Gmail predicts which emails might be of high importance to you so it marks them as important with a yellow icon. If your inbox ends up full of so called important emails, this is how you disable this feature.
If you don't have installed program for editing PDF documents on your computer, then you can use web application PDFescape which allows you to edit your documents in your browser.
In Gmail there is no insert/upload image option by default. Let me show you how to enable the option so you can upload and insert an image into the email or insert an image by providing url.