GitHubが用意してくれている。ありがたい。
https://github.com/github/gitignore

Railsならこんな感じ。
https://github.com/github/gitignore/blob/master/Rails.gitignore

Rails.gitignore

*.rbc
capybara-*.html
.rspec
/log
/tmp
/db/*.sqlite3
/db/*.sqlite3-journal
/public/system
/coverage/
/spec/tmp
*.orig
rerun.txt
pickle-email-*.html

# TODO Comment out this rule if you are OK with secrets being uploaded to the repo
config/initializers/secret_token.rb
config/master.key

# Only include if you have production secrets in this file, which is no longer a Rails default
# config/secrets.yml

# dotenv
# TODO Comment out this rule if environment variables can be committed
.env

## Environment normalization:
/.bundle
/vendor/bundle

# these should all be checked in to normalize the environment:
# Gemfile.lock, .ruby-version, .ruby-gemset

# unless supporting rvm < 1.11.0 or doing something fancy, ignore this:
.rvmrc

# if using bower-rails ignore default bower_components path bower.json files
/vendor/assets/bower_components
*.bowerrc
bower.json

# Ignore pow environment settings
.powenv

# Ignore Byebug command history file.
.byebug_history

# Ignore node_modules
node_modules/

これをベースに、環境やプロジェクトに応じたものを作っていけばよさそう。

OSやエディタについても、メジャーなものは大抵用意されている。
https://github.com/github/gitignore/blob/master/Global/

例えば、Macユーザーなら毎回.DS_Storeを追加しているはずだが、Mac用の.gitignoreもある。
https://github.com/github/gitignore/blob/master/Global/macOS.gitignore

macOS.gitignore

# General
.DS_Store
.AppleDouble
.LSOverride

# Icon must end with two \r
Icon


# Thumbnails
._*

# Files that might appear in the root of a volume
.DocumentRevisions-V100
.fseventsd
.Spotlight-V100
.TemporaryItems
.Trashes
.VolumeIcon.icns
.com.apple.timemachine.donotpresent

# Directories potentially created on remote AFP share
.AppleDB
.AppleDesktop
Network Trash Folder
Temporary Items
.apdisk