jQuery Rain gives the latest list of their 'nominations' for the 100 best jQuery plug-ins.
Some you'll recognise, some you won't, many you'll want to try.
A couple that I'll be looking at seriously are FlowType.JS, which responsively adjusts font size and line height based on the width of the containing element, and slimMenu, which creates responsive menus on the fly (in a similar way as I did in one of my responsive menu demos).