Here is the formula to calculate the difference between two dates in Salesforce. The difference is returned in Days, Hours & Minutes. Works like a charm. More
Here is the link to cheat sheet for some useful validation rules that you can incorporate in Salesforce.com. More
Use Dataloader.io to import / export data from Salesforce.com. What I specially liked about Dataloader.io is that it is entirely web based. Unlike Salesforce Apex Dataloader, you don’t need to install anything on your computer. More
If you want to change the custom field of ‘Text’ type to ‘Picklist’, the new picklist field will automatically pickup current distinct values from the text field (upto a maximum of 250).