There are a few tools for doing really easy 3D modeling. SketchUp (http://sketchup.google.com/) is incredibly easy to use and can have you building levels in minutes. Its only drawback is it draws very straight geometric shapes. If you want to sculpt more organic shapes then check out Sculptris (http://www.sculptris.com). For a fully featured free 3D modelling package you have to check out Blender (http://www.blender.org), historically this was hard to recommend with its very obtuse interface but that’s all sorted now and it’s a much nicer tool than it’s ever been.
If you’re happy to drop in pre-made 3D assets check out Turbo Squid (http://turbosquid.com) and The 3D Studio (http://www.the3dstudio.com) these sites are full of both free and paid-for models created by 3D artists. There’s a lot of great stuff in there you could just drop right into your game.