A key point was made by one reference: "Some of the files have errors in them and you cannot open them up directly. The workaround is to import that After Effects project file into an existing project file."
Of course most tutorial authors offer free projects and presets, but there are a few additional resources for projects and presets:
- Video Copilot offers many free goodies
- XScriptorium has a searchable database of AE scripts, expressions, presets and projects
- AE Enhancers, the uberforum for scripts & expressions, also has Presets Library and there's more throughout.
- Forging Fire's log of external tutorials
- Scott Frizzle has freebies
- AE Freemart has a collection
- Adobe's After Effects Exchange has user uploads and more from Adobe, like Additional Presets for After Effects CS3 (for Shape Layer and per-character 3D text).
For a video tutorial on working with effects, Animation Presets, & Brainstorm, check out Chris Meyer's take on CS3 at the Adobe Video Workshop, and a newer intro in After Effects Apprentice Video Tutorial #2 (Taking advantage of the Effects & Presets panel). BTW, to prevent Presets or filters (or folders of them) from loading, you can add “~” or use parentheses () on both Mac & Windows.
Also, there's a tutorial by Ko Maruyama on creating previews for AE animation presets for Bridge: Previews for AE animation presets. And of course, there's presets mentioned in AEP posts labeled presets or Animation Preset.
Update: AEP Project is a website made for Adobe After Effects Japanese users that publishes projects, presets, resources, tutorials, and many another tips. AE.Info is a similar effort from Japan. For additional resources on presets see After Effects presets & templates burgeoning.