Workbook
http://www.salesforce.com/us/developer/docs/workbook/index.htm

Fundamentals
http://www.salesforce.com/us/developer/docs/fundamentals/index.htm

Cookbook
http://www.salesforce.com/us/developer/docs/cookbook/index.htm

Apex Advanced Code Example
http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_shopping_cart_example.htm
https://sites.secure.force.com/appexchange/listingDetail?listingId=a0N30000001saDCEAY

And many more to come!

Lookup filters function similarly to validation rules when you save a record. That is, actions that cause related records to save, such as changes to a roll-up summary fields, also trigger the lookup filters on the related record and block the save.

The implications for this are massive. Let's explore two examples:

Example 1: Filter as Validation Rule from Parent Record

• We create a lookup on a Child object to Parent.
• We filter the lookup to EXCLUDE Parent.Status = 'Closed' (Parent.Status is only Open or Closed.)
• We can edit the Child records as long as the Parent Status is not Closed.
• When Parent.Status is changed to Closed, existing related Child records are not affected...
• BUT if we attempt to edit a Child when the Parent is Closed, Force.com will throw an error (which we can customize) beause that the Lookup is invalid.
• (and clearly we cannot add new Child records either)

Conclusion: Thus, Filtered Lookups act much like Validation Rules. A quick experiment shows that Filtered Lookup errors actually fire before Validation Rules.

Example 2: Filter as Validation Rule on Roll-Up Summary (from Child Record) - what the line above was referencing

• Use the above example, but change the lookup to a master-detail relationship
• Create a Roll-Up Summary field to count all child records
• Prevent saving more than 10 child records for one parent record

Here, we have triggered a filter error without touching a parent record, yet we throw an error based on a value on the parent record.

This second example is significant because we could already prevent more than 10 child records from saving, but doing so required a Roll-Up Summary field on the parent object AND a Validation Rule on the child object. Now we can replace the Validation Rule with the Lookup Filter, though we still need the Roll-Up Summary field. Whether or not this simplifies things is definitely up for debate...

Conclusion

This is a very powerful feature! Thanks to salesforce.com for rolling it out, even in beta form.

Real world example: The above example would be great for Time Sheet Entry and Time Sheet Header objects, as they would create, in effect, a validation rule on the Header record preventing editing of any child records. Awesome!

For further reading, check Salesforce Help's Lookup Filters examples.