Archive
SP.ClientContext.get_current is null or not an object
One of the reasons you might be getting this error is because you are trying to use the client object model without the sp.js file being loaded.
Other post regarding errors you might be getting related to the client object model and sp.js
You might need the ExecuteOrDelayUntilScriptLoaded method
Uncaught TypeError: Cannot read property ‘get_current’ of undefined
Populate a drop-down list using SharePoint Client Object Model
This video will demonstrate how to populate an HTML drop down list control with SharePoint list column values. I’ve created a visual web part in Visual Studio 2010 and used the client object model to demonstrate this. Why would I need to use the client object model? When developing for sandboxed solutions, the whole server object model is not open for use. In some cases, such as developing for SharePoint Online, client object model will be useful if not absolutely necessary. However, you can populate a drop down list with the server object model if you prefer.
Client Object Model: SP.UI.ModalDialog.showModalDialog(options)
SharePoint 2010 has introduced a client object model. Now you can use JavaScript to access SharePoint data. Visit MSDN to have a look at the JavaScript API
http://msdn.microsoft.com/en-us/library/ee538253.aspx
Create a SharePoint 2010 list using JavaScript client object model
Create a SharePoint 2010 list using JavaScript client object model. Get the .wsp here.
SharePoint (and Project Server) Shenanigans 