Open Source

C# / .NET

• DynamicBuilder Suspiciously pleasant XML construction API for C# 4, inspired by Ruby's Builder
• Nate Simple little fluent state machine for C# that doesn't have its way with your architecture.
• ArgumentReflector Spoonful of syntactic C# 3.0 sugar for accepting named, optional, method parameters via anonymous objects.

JavaScript

• DeLorean Flux capacitor for accurately faking time-bound JavaScript for testing purposes, including complex interactions of timeouts, intervals, and dates.
• Pavlov Rich, Behavioral, API over QUnit

jQuery

• jQuery.lifesupport Smart, simple, and secure session management plugin
• jQuery.safetynet A smarter in-browser "unsaved changes" warning plugin
• jQuery.autotype Simple, accurate, typing simulation plugin
• jQuery.netchanger Plugin enabling a rich extension to the DOM onchange event
• jQuery.flextarea Smart HTML5-aware auto-resizing textarea
• jQuery.fasten Slightly smarter fixed positioning plugin
• jQuery.confine Textarea maxlength, Done Right
• jQuery.updatepanel proxies ASP.NET AJAX client-side events as native jQuery events via the jQuery Special Events API.

Other

• ColorScrub Simple pure-javascript tool for quickly viewing and sharing a local or remote color palette
• Monokai-VS Monokai color theme for Visual Studio 2010 and 2008