Visualforce - Beyond Basics Series 5 - App agnostic widgets in sidebar

Is there a customization section common to all Apps ?

Yes. Salesforce provides sidebar for this purpose. This sidebar is nothing but a collection of standard & custom components. Standard components include Create New, Recent Items, Tags, Custom Links, Recycle Bin.

What is allowed inside a sidebar component ?

you can have one of the following types for a component,

1.   Links
2.   Image
3.   HTML
4.   Visualforce

How can we use visualforce ?

Most of the time one can use visualforce to build a sidebar component for one of 3 purposes,

1.   Output Stats, Quotes, Reminders, Important messages etc
2.   Input a value from user & take action in Apex
3.   Set of links for Navigation

Where is the code ?

I have given code for illustrating all these use cases. To try this out,
  1) you need to create a new 'Custom Component' under Setup -> Home -> Home Page Components.
        - with 'visualforce' type, height 200px, narrow type
        - select the visualforce page (copy the code from below)
  2) Then, you need to add this new component into current Home Page Layout.