I've put together what is my current stack of what I consider quality development tools. I've put them all on a server in a file called GOAT (Greatest of all time). It works like a personal repository for my favorite frameworks and plugins.
Here they are:
bootstrap (aka Twitter Bootstrap 2, use bootswatch.com to get a good theme.
bootstrap.colorpicker (Color picker for Bootstrap)
bootstrap.datepicker (Date picker for Bootstrap)
jquery (A no brainer...)
jquery.chosen (Nicely style SELECT elements for easy filter)
jquery.noty (The best jquery notifications plugin out there)
jquery.qtip2 - Excellent Tooltip Library
jquery.tinycon - HTML5 Favicon Manipulation Library
jquery.mobile (Slowly becoming more stable, very neat stuff so far)
jquery.mobile.bootstrap.theme (A great theme for jquery mobile)
jquery.mobile.gmap - Responsive Google Maps for mobile devices
jquery-ui.bootstrap (The bootstrap theme for jquery UI)
jquery-ui.smoothness (A simple, clean theme for JQuery UI)
knockout (Knockout.js is simply amazing MVVM. Do their tutorial and you'll see why.)
FuelPHP (FuelPHP is the library I've settled on for doing my MVC and so far I've very impressed)
Foundation by Zurb (I want to use this, but Bootstrap 2 make is tough. I keep going back and forth)
jquery.tokeninput (Token input is nice but development has stopped. In the end, I went with jquery.chosen instead.
That's all I've got for now. I will try to update this as I add to it. Would love to hear suggestions or potential additions to this list of core dev tools/frameworks for web application developers.