Michael Höhne describes a lot of interesting things about MS CRM 3.0 and 4.0. It is mainly about scripting, and less about customizations. http://www.stunnware.com/crm2/
Below you will find a list with examples of javascript code you can find on the Stunnware page.
(Since I will not repeat his work, you have to go to his site).

More JavaScript Code

Replacing the content of an IFRAME
Adding additional values to duration fields
Changing the title of a CRM form
Changing the Link of a Ticker Symbol from MSN Money to Yahoo Finances
Counting the number of backslashes in a string
Changing the label of a field at runtime
Changing the color of a single option in a picklist
Opening a new window without the IE menu and status bars
Retrieving the text of a lookup control
Disabling a form at runtime
Automatically changing the value of a text field to capitals
Getting notified when the user selects a another tab on a form
The current record id
The current object type code
Initializing a date field with the current date
Changing the URL of an IFRAME at runtime
Displaying a picture in a CRM form, using a text field to store the data source
Changing the default lookup type
Checking if a field is present in a form
Calculating a date field based upon the selection of a picklist
Creating a new email when double-clicking a standard text field
Resetting a field value if validation fails
Inserting line breaks in a text area control
Hiding an entire row of a CRM form

More JavaScript Code – Part 2

Disabling the time selection of a date/time field
Changing the time interval in date/time fields
Getting the quote id inside a quotedetail form
How to know which button triggered the OnSave event
Accessing the CRM database from JavaScript
Modifying the Quick Create Form
Forcing the selection of an account in the potential customer field of an opportunity
Hiding and showing fields dynamically based on other field values
Removing the value from a lookup field
Hiding tabs at runtime
Setting the active tab
Displaying related entities instead of the form when opening a record
Modifying the color of disabled form fields
Removing a navigation bar entry at runtime
Changing the color of a label

More JavaScript Code – Part 3

Setting the background color of a CRM form
Why can’t we use VBScript to write client-side code?
Some calculated fields on the CRM form are not saved in the database
Changing detailed tooltips when hovering over fields in a CRM form
Fixing problems with orphaned OnChange event handlers
Setting the field required level at runtime
Setting a default time in a date field
User interaction with yes/no style message boxes
Testing if a lookup field has a value
Comparing date values
Getting notified when the user enters a form field
Overriding the click event of a lookup field
Formatting a date to YYYYMMDD
The DataValue of a picklist is a string!
Setting a picklist’s default value in code
Starting an application from a CRM form
Changing the available entity types in a lookup dialog
Setting the text of the “Save As Completed” button
Calculating durations
Hiding a single field
Setting a text field to the name of the entity referenced in a lookup control
Getting notified when a user changes a checkbox value before leaving the field
Preventing an OnChange event handler from executing when the form closes

Client Side Scripting – More JavaScript Code – Part 4 

Maximizing a form
Using a toolbar button to open a referenced entity
Calculating the sum of two or more fields
Calculating the total charge based on actural duration, hourly rate, trip charge and tax rate
Changing error messages in CRM forms
Setting a custom date field to another date minus 60 days
Changing the default height of the lookup window
Accessing the previous field value in OnChange
Automatically calculate the tax value in an invoice line (invoicedetail)
Performing an action when a CRM form closes
Hooking into the “Lookup Address” feature in the order form
Changing the form title (not the browser title)
Passing parameters from a toolbar button to a CRM form
Aborting an OnChange operation

Client Side Scripting – More JavaScript Code – Part 5 

Formatting international phone numbers
Rounding numerical fields
Be aware of null values in Boolean fields
Reusing code in OnLoad and OnChange event handlers
Getting notified when the user selected an address in the address picker (quote, order, invoice)
What is “if (condition) ? statement1 : statement2″?
Copying the display name of a selected lookup value into a textbox
Retrieving all fields inside a CRM form
Knowing if you are running in CRM 3.0 or CRM 4.0
Showing/Hiding tabs based on the selection in a picklist
Changing the background color of a form (CRM 4.0)
Calculating the difference of two numerical fields
Disable all fields on a form
When events do not fire anymore

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