You should start using Cyberduck, it has a Free Client which pops up a window once you installed the free version (but the popup appears only ONCE and does not contain any malware or advertisments), and a paid version without a popup.
Actually, by default, the popup appears every time you close the app, but there's a clear option to suppress it on subsequent occasions for 'this version'. I've never selected that option, so I'm not sure how fine-grained 'version' is; I actually like seeing the reminder because I will, definitely, one day, make a donation, so I appreciate the prompt!
still it only advertises itself, not any other software.
that's why i would definitly trust cyberduck.
And Cyberduck also supports Rackspace Cloud Files and Amazon S3 containers.