How to create custom hyperlink field in Salesforce

Salesforce.com allows you to create a custom field with type URL. However, sometimes you may want to create a custom URL for your special requirements.
For example, you have a separate application call Human Resource Management (HRM)
Your Salesforce application contains a custom field call EmployeeID__c in Contact object
Contact tab layout:
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Contact name | EmployeeID| E-mail | ...
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Thao Truong | 000111 | abc@yahoo.com |
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John Peter | 000112 | abc@gmail.com |
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When user clicks on EmployeeID cell, you would like it can be linked to your HRM application with the link like that:
http://YourHRM/Employees.aspx?ID=000111
or http://YourHRM/Employees.aspx?ID=000112

In this case, you could create a new custom field EmployeeIDURL with Formula type
Formula syntax:
HYPERLINK("http://YourHRM/Employees.aspx?ID=" + EmployeeID__c, EmployeeID__c, "_blank")
That's all.