For example, to create or edit an object, when I want to add or change some value to one specific field, I need to refer another website to retrieve some info.In this way, may I add one button or link on the edit page?
The help link of SalesForce is here: https://help.salesforce.com/apex/htviewhelpdoc?id=co_edit.htm&language=en_US
Thanks.
Custom buttons and links don't display on new/edit page layouts so even if you'd make one, it will be visible only on detail view.
To use method #3:
<script>document.getElementById('Name').disabled = true;</script>
. You need longer script though to somehow create an URL at runtime and append it somewhere on the page near your "special" field.<script src="filename.js" type="text/javascript"></script>
I'm not too proud of this hack and if you can - go with custom Visualforce new & edit page. It's quick & dirty, probably should be used only when you're not allowed to override pages.. but it works.