Demoscene and free software
For a couple of years, I've been active in both the demoscene as a member of the group Headcrash and organizer of the 0a000h-party and in the free software movement as a developer of Gentoo Linux (free software means the freedom to use, copy, modify and redistribute the software, thus it's more than just freeware). While both scenes are very similar in their behaviour, their interests and their skills, there are very few people who are active in both scenes and cooperation is very rare. I think both scenes could benefit much from each other.
Make your productions cross-plattform
The demoscene today is mostly using Windows as the platform. There aren't many reasons why demos couldn't be written cross-plattform. While it's a neverending discussion if DirectX or OpenGL is better, they don't differ that much. OpenGL is cross-platform, but still many OpenGL-Demos are written for Windows only, because they use proprietary, Windows-only OpenGL-"extensions" (Microsoft's usual behaviour to undermine standards), because their sound-system is not designed for cross-platform use (take libSDL or make some #ifdefs) or they just never thought about making their demo cross-plattform. Many Linux-users (like me) haven't even installed Windows, so they are excluded from viewing demos, especially as there are no things like live-CDs with Windows.
Linux productions on demoparties
At the last Breakpoint, the biggest demoparty out there, Linux entries weren't accepted in the compos. I couldn't really see a reason for this. While it might be true that at previous Breakpoints, there weren't many linux-entries, this is completely different at other parties. For example, at Assembly there are always a bunch of Linux-entries, sometimes even winning compos (remember "Yellow Rose of Texas"?). The Breakpoint-organizers also complained about the trouble it would cause to allow Linux-entries. Guys, there are Linux-livecds out there, they don't require any installation. Have a look at the Games Knoppix, it already comes with 3D-drivers enabled. For the required libraries, just say "it must run on latest Games Knoppix" and you won't have any problems. I really doubt this is more trouble than installing Windows XP.
Demoscene skills for free games
Most demogroups don't release the source codes of their productions. There is no real reason for this, most people already give their sources away if you ask them. If people want to protect their sources because they want to re-use the technology in other productions and don't want competing products to use it, they could release it some months afterwards.
Often if I ask people why they don't use Linux, they tell me it's about the games. Free games are rare, the demoscene has the skills to create them.
Creative people for free Content
Creative Commons and other initiatives for releasing media content under free licenses are quite successful at the moment. Examples are Wikipedia, a lot of music productions (Linuxtag-CD, Wired-CD) and even some movies (Route 66, CH7), which were released under Open Source-like licenses for media content.
The demoscene has a lot of creative people. The music in competitions is sometimes really great, my desktop-background was often a demoscene-production. Why not sharing this content with others and release it under a Creative Commons license or similar? Usually, demoscene-releases are considered freely redistributable, but the legal status is often uncertain, platforms like scene.org have already had problems with this. Creative Commons is a set of licenses that allows you to decide what can be done with your work, e.g. you can define if it may be modified. It definitely grants the right to freely redistribute your stuff.
Digital rights management
Large parts of the hard- and software industry are planning a technology called "trusted computing" for the future to implement Digital Rights Management (often called Digital Restriction Management). This means that a chip in the computer (or later the hardware itself) checks if a software is allowed to run, can access a media-file and similar things. Such technologies are also a big threat to the demoscene. It could mean for example that you aren't allowed to distribute your own productions on other computer systems or create your own software at all without a license.
If you think this won't happen, it already has. For example, see the xbox, you aren't allowed to put your own software on this system. You can do it, but only because the protection of the xbox is buggy and was hacked a while ago. Microsoft's plans were that no one could install software on it without having it licensed by them.
Spreading free software is the alternative to so-called "trusted computing" and DRM-technologies. It can't coexist, free software always gives you the freedom to do with your software whatever you want.
Idea: Optimize Emulators for Demos
As most demoscene productions are still for Windows, a linux-only-user like me is excluded from viewing them. Running demos in emulators like WINE or Cedega isn't working very well. An idea might be to create a project that optimizes WINE (and also Dosemu/Dosbox for the good old times) for demos.
Conclusion
The demoscene and the free software movement are imho both great. At the moment, they do not have much in common. I'd really like to change that.