Good for avatars and such!
Input file fields in HTML forms are notoriously ugly and inconsistent across browsers.
ImageDrop is a tiny (989 characters), deceptively simple, and entirely effective way to make file input fields consistent looking while adding an image preview and placeholder text.
ImageDrop employs a little CSS trickery to change the file input field into a drop zone.
During hover, a div is temporarily shown to act as the placeholder.
The html structure in the form looks like this:
<div data-imagedrop> <input type='file'> <div>Placeholder</div> </div>
And, yes, that's completely compatible with bootstrap.
ImageDrop has been packaged as a gem, but the raw js and css files are included for use outside of rails.