I am using this query in a custom report (CMS Desk > Tools > Reporting). It's to populate a dropdown form control in a parameter. I think you have solved my problem, though.
I misunderstood how joins worked. For some reason I thought fields had to have the exact same name in order to map to each other, but apparently they don't :)
I did modify the query that you provided, though. Here's what I changed it to:
SELECT DISTINCT CMS_User.UserName, CMS_User.UserName
FROM customtable_time_tracking JOIN
CMS_User on customtable_time_tracking.ItemCreatedBy = CMS_User.UserID
WHERE DepartmentLabel = 'Web Development'
I was trying to get the UserName field from the CMS.User table and populate my dropdown with a list of all of the users whose UserID matched the ItemCreatedBy field in my custom table. Just changing the SELECT statement made everything work fine.
Thank you for your help :)