Is there any widget that allows uploading zips or multiple files at once?
One possible module is Plupload integration; however, it seems to be unstable and is under heavy development.
Are there any well known stable multi upload widgets?
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If you are uploading images and on Drupal 6 (your post doesn't say) I have hade good results with Image FUpload
I've been using Plupload on a Drupal 7 site for about a year with no stability issues I can recall.
I chose it after testing most of the other options I could find and it was the most stable and simplest to use. As I say, that was a year ago and a lot may have happened since then, so others may have surpassed it or they may fit your use case better (some others were good, they just didn't meet my needs as well).
The only "issue" I have is that the form when you have 50 images with alt, title, etc in your form and in those cases things really bog down when you edit the page and on slow connections I have had timeouts when I save the page. I'm not really sure how you would get around this though unless the image manage/edit/upload actions were distinct and separate from the page edit/manage/create functions and possibly separate from each other (manage/edit versus upload). Short of that, you're going to have pretty heavy forms no matter how you do it.