Silverlight is Microsoft's browser-based rich internet application that allows for animation, vector graphics and video playback etc and as such competes with Flash and the presentation components of Ajax. It also competes with Sun Microsystems' JavaFX, which was launched a few days after Silverlight.
On balance (for something created by Microsoft) it has been well-received by the developer community. Some have even called it the "Flash-killer".
It just goes to show that there is not a safe laurel to sit on in the world of Internet development.