[SalesForce] Sorting related-list records

Try to use the following formula of type "Text":

IF(ISBLANK(FieldToSortAfter__c),  IMAGE('/s.gif',' ') , ' ' + TEXT(FieldToSortAfter__c))

The formula checks for the blank value and if it is then it puts a transparent image so nobody can see it on the page. But if the field has a value then just puts a space before it so the sorter "thinks" that this value is "smaller" that the image :)

Here is my result:

Currently its not possible to sort open activites.Here is the idea https://success.salesforce.com/ideaview?id=08730000000BqX6AAK

Workaround : Create a list button on Task

with URL value as /007?id={!Account.Id}&rlid=RelatedActivityList

Add the button to Open Activities Related list of Account Pagelayout.Then on Click of button it will open new page where you could sort open activities on any field.