Client side web is quite new for me. Last year was interesting in this area. Let me list some tools:
Bootstrap, from Twitter - this is killer. You add link to precompiled CSS file in your page and it starts look just well. For utility sites where you do not want to spend time on design this is mast have. Another related project, jQuery UI Bootstrap, adds Bootstrap look and feel to jQuery UI.
Knockout - magic toolkit that brings MVVM for you. As for me too much magic. But overall is very nice. Thanks to Roman Gomolko for introduction. Also some people suggested that Backbone.js is similar but with less magic, so this is something I will try this year. Another library I need to try is RxJS.
WCF Web API - next generation of WCF that will make HTTP stuff more user friendly. It quite unstable(it’s preview version), also Glen Block is not on it anymore, so I cannot say that everything is light. However I really like it and use it in some of my pet projects with success.
ELMAH - have to point this tool, but I do not really like it. First of all it look like alien in ASP.NET MVC world (fix). Second it is not useful. There is no daily digest, no filter on UI, no grouping or another stuff that could help to analyze errors. This is another time I am thinking about another pet project.
Something like this…Comments.