$/
  docs*/
  src/
  tests/
  samples*/
  artifacts/
  packages/
  build/
  .gitignore
  .gitattributes
  {solution}.sln
  NuGet.config
  build.cmd
  build.sh
  README.md
  LICENSE

• src - Main projects
• test - Test projects
• docs* (optional) - Documentation stuff, markdown files, help files etc.
• samples* (optional) - Sample projects
• artifacts - Build outputs go here. Doing a build.cmd/build.sh generates artifacts here (nupkgs, dlls, pdbs, etc.)
• packages - NuGet packages
• build - Build customizations (custom msbuild files/psake/fake/albacore/etc) scripts
• build.cmd - Bootstrap the build for windows
• build.sh - Bootstrap the build for *nix

[Oo]bj/
[Bb]in/
.nuget/
_ReSharper.*
packages/
artifacts/
*.user
*.suo
*.userprefs
*DS_Store
*.sln.ide

There's probably more things that go in the ignore file.

• Update: Added docs folder
• Added README.md and LICENSE - Critical if you're OSS, if not ignore it
• Renamed test to tests