Bucket Explorer has released latest version 2012.03 last week. This version introducing Storage Calculator
an awesome feature of Bucket Explorer, which helps you in identifying Bucket/Folder Size of S3 Buckets. It represents the files and folders of the selected bucket in tree view with its occupied size at Amazon S3. You can identify size for Bucket, individual folder as well as file. Tree view helps you to list folder at any level easily and faster than tabular representation. This calculator also keeps details of all calculated Buckets for storage. Information about each individual bucket is listed in table. From here you can view again in tree format by double click on the row for the desired bucket. This calculator also keeps information in addition to storage like Bucket's location, folder-count & file-count.
Try its 30 days trial version download from here