Recently, Amazon S3 introduced a new multi-part upload feature.
You can now break your large files into parts and upload a number of parts in parallel. If the upload of a part fails, you can simply restart it. You can improve your overall upload speed by taking advantage of parallelism.
The multi-part upload feature is enabled by default in S3 Browser Free and Pro. All files larger than 5 Mb will be uploaded via multiple parts in parallel.
The multi-part download feature is also supported by S3 Browser Free and Pro!
But as you already know, the Free version is limited up to the 2 simultaneous transfers.