Squid

Most game developers write their UI from scratch. Often enough it’s not very reusable and will provide a serious challenge to adapt to changing needs. Our solution to this dilemma is called SQUID.
SQUID is an SDK to create user interfaces for games and other 2D/3D realtime applications, using a Retained Mode system. It does not depend on a certain rendering engine, you can use it with any engine you like, on any platform that supports the .NET 2.0 framework.
SQUID SDK is written in managed C# and comes as .NET 2.0 class library for easy integration into your projects.It is completely free. Reference it, use it, it’s that simple!

Check out the News | Browse Documentation | Download Squid

SQUID is a GUI blackbox

SQUID is easy to integrate

All you need to do is to implement 1 compact interface, the ISquidRenderer.

it is engine independent
it does not manage textures or fonts, only integers and strings
it doesn’t draw anything, you do

To get you kickstarted, sample implementations of ISquidRenderer are included:

XNA 4.0
SlimDX
TrueVision3D
SharpDX (soon)
MonoGame (soon)

Features:

Standard controls:

scale9 texture grid!
docking, anchoring
margin,padding
z-order, clipping, scissoring
hierarchical opacity
input event handlers
international keyboard support
custom mouse cursors
tooltips

drag & drop
snapping windows
modal windows
control state fading
easy skinning via styles
texture UV mapping
full scissoring control
automatic batching
it’s fast!

Button
CheckBox
DropDownList
Dialog
FlowLayoutFrame
Frame
ImageControl
Label
ListBox
ListView
Panel