To start with, I became a huge fan of ptototype.js couple of years ago when I first checked it out (perhaps 2007), why I love it is because I am a coder, parallel framework was Script.aculo.us, it looked good too. Lately, Yahoo UI also has created lots of buzz.
But those extensively working with frontend development and are looking to add just the appealing frontend techniques do not have to code much, they can look for mootools & jquery, which are highly popular for their library of frontend widgets.
Dojo Tool Kit, offers both, coding level libraries and ready to use widgets.
There something recently has caught my attention and that is OAT Framework, it has nice Tabular Grids / Pivot Table / Tree Control / Timeline / SVG Based RDF Graph, to such extent that I am fully impressed being a database guy.
Qooxdoo has some nice ready to use examples including twitter and HTML Editor.
If you are building complex dashboard like applications, recommend you check out extjs (paid).
Rico, has nice weather widget which you may want to check it out.
cross-browser has some nice cross browser animation examples, for more visual effects check out Script.aculo.us, MooFX, Bytefx, Prototype Windows, Lightbox, Lytebox, Leightbox, GreyBox, Behaviour, Curvy Corners, Nifty Corners, jsScrolling
Also check out some other tools for your development / optimization, JSlibs, DOJO shrinksafe – compression, JSmin – compression, JS flash version detector, Open JSAN, MiniAJAX, Google trends of the major players, Smooth Gallery, Slideshow, Flickrshow
Some tools for specific requirements XML.ObjTree XML parser, jsvalidate, Form validation, Store json data in cookies, Livepipe Tabs, BSN AutoSuggest, BarelyFitz Tabs, overLIB Tooltips, Kryogenix Sortable tables, FD Sortable tables, JDV Sortable tables, TableKit
Hope this will help us build the next generation apps for the generation now and generation ahead!