Hi and welcome to OGA! :D

Almost anything is welcome 3D wise as long as it could reasonably fit in a game. Same for music and sound effects. We are interested in concept art, story images and portraits. I for one would certainly like to see more high-spec in-game stuff like eg. http://www.bit-blot.com/aquaria/media.html. In short, pretty much everything you mention is useful. As for what direction to take with subject and style, that is really up to you and your preferences, but we have some loose collaborations going on if you want direction without too much commitment (a Diablo style dungeon crawler by pfunked, helping http://radakan.org, a japanese-style RPG in an early engine stage).

If none of that interests you, feel free to tell us what you would like to work on :)

2D:

Bigger cohesive sets of portaits are quite cool.

Menu backgroung graphics (modern and medival, f.e. some pergament scrolls) are always nice to have.

2D Icons for varios stuff (preferrably vector graphics though).

3D:

Sets of animations

everyday items for mapping (especially usefull would be street props I guess, f.e. street lights, speed bumps, poles etc... also medival settings etc)

Well, you asked us :-)

I think most people out there looking for free 3D arts are programmers like me who just are not skilled enough to create arts good enough for demos. As long as you don't have something very nice and impressive to show it's hard to find artists helping you out of your dilemma. Without artists a programmer can hardly produce anything visually impressive and this can be quite frustrating.

That means some generic textured models with standard animation sequences (idle, walk, run, etc.) are highly welcomed. Then of course some trees and buildings to make a scene less dull. Most 3D game projects will need something like that to test animation code and create demos - except for those lucky ones who can do that on their own.

I cannot speak for 2D projects as I haven't done one in years.

It is great you asked :), but to answer your question is a little bit hard.

We have a poll about what is needed, but I don't think that it is really representative. Probably it is best if you create the art you like most and are the best at as long as it might be useful for games. Another option is like ceninan said to help one of the loose collaborations of ours like radakan or pfunkeds game. If those answer are not enough for you feel free to join us in IRC on #opengameart at irc.freenode.net and discuss it with us directly. There are always people asking for some stuff and it is probably quite easy to arrange some loose collaboriation that fits your wishes. You don't need to get involved much in a specific project this way and your art can help a game directly as well as being useful for a wider audience through Opengameart.

As for 3D models, I can tell you that low-poly but good-looking trees (with alpha textures) are highly reusable in nearly any kind of game, be it medieval, modern, racing, fighting, FPS, whatever.

But of course, make what's most fun for you to make, as it's what will look best ;)