The 'other' plug-ins
Adobe's 'other plug-in', Shockwave, looked briefly like it would realise 3D on the web, but its authoring software director is no longer even listed as a product on the Adobe homepage.
A more interesting one to watch is Unity. Primarily aimed at game development, it looks to ease the creation process using its proprietary editor and deploy to platforms including the web, Wii, iPhone and standalone applications.
Finally, Silverlight is a direct competitor to Flash that opens up rich media development to C# developers and reputedly has some performance advantages. While Microsoft claims 300,000 developers are targeting the platform, the creative industries aren't adopting it in a hurry. However, Microsoft isn't going anywhere, and it may only take one new killer feature, or support on a popular new device, to change the game.
Plug-ins vs open standards
Plug-ins pose a number of problems, not least those of accessibility and SEO. They require installation and the software is invariably proprietary with varying degrees of cross platform support. The web is becoming more than just cross-platform, it's becoming cross device, so plug-in dependant content is becoming more of a problem.
Particularly notable is the absence of Flash support on the iPhone. New platforms and devices are going to implement open standards over proprietary plug-ins and there'll be greater pressure (and incentive) for the big companies to play along.
It's tempting to reach for Flash every time, but don't underestimate what can be achieved with dynamic HTML techniques. If all you need is a fancy slideshow with scrolling or fading effects, you may be able to achieve your goals without touching Flash.
The level of dynamism users expect to see even in HTML content is much higher than just a couple of years ago.
HTML 5 & the Canvas element
HTML 5 introduces a number of new elements for rich media support, including audio and video playback and the Canvas drawing API. It's only a draft spec, but it's already seeing adoption in some browsers as well as devices like the iPhone and on the Android platform.
The new Canvas element essentially provides a means to dynamically draw interactive graphics to a web page without using a plugin. The standard even has an allowance for a 3D context probably based on OpenGL. So one day we may see native 3D running in browsers across all HTML 5-capable devices.