I love Dropzone. I know I'm incredibly late to the party as it has been around for years, but I still want to profess my love for it.
I think I first learned about it from some podcast that Brett Terpstra was on, and I bought it pretty much right away. Then I forgot about it forever until I needed a nicer way to do scp upload combined with opening a file on my home server than opening up a terminal.
After having gone through their very excellent API documentation I built my first version of UploadOpen in 5 minutes, and after another 5 minutes of debugging it worked perfectly exactly like I wanted with messaging etc.
With this success done I built scripts to upload images to my new moln.is server, scripts to paste to paste.pm and.. I'm still going.
This is something I've wanted for a really long time, a nice and easy way to put a basic UI on shell scripts, ruby scripts etc. Being able to process a file by just dragging it to a circle on the screen is so very nice and while I could build my completely own solution for it this saves me so incredibly much time.
I posted one script to Github, and I'm going to keep adding my scripts as I finish them hoping they may inspire other people.