Amazon S3 storage is quickly becoming staple for having some files in cloud. It's reliable, affordable and big brand. I have to deal with it more often in context of WordPress backups and it is kinda inconvenient to just get file downloaded to your computer from S3.
S3 Browser is actively developed desktop client for managing files in S3 buckets.
What it does
S3 Browser takes your Amazon keys and connects to your account. Available buckets are automatically listed and clicking through them loads lists of contained files.
App gives you full range of file manipulation (upload, download, delete) and can manipulate both files and bucket themselves.
Supports bulk operation, that is really nice when you need to get a lot of stuff up or down.
Aside from straight file manipulation S3 Browser also focuses on giving you access to many specific options, from managing buckets and user properties to configuring static web site hosting.
App really, really, really likes to nag about Pro version - which allows commercial usage and has range of advanced functions, as well as more snappy implementation of transfers.
I suppose it's effective at selling to people who need more features, but for occasional basic usage it is annoying.
App works nicely, has plenty of functions, native portable option and is very actively developed. The downside is heavy nagging and upselling in free version.