Here’s the complete list of packages I have installed, excepting those that I work on myself.

Here are some particularly useful packages.

Package Description
AdvancedNewFile File creation.
ApplySyntax ApplySyntax is a package for Sublime Text 2 and 3 that allows you to detect and apply the syntax of files that might not otherwise be detected properly.
auto-save Automatically save files after every edit.
Bracket Highlighter Bracket and tag highlights appear in gutter, also provides easy navigation and selection of content within brackets.
Emmet Awesome shortcuts for writing HTML and CSS.
FileHistory Keep track of files which have been recently closed, as well as files that have been recently accessed.
GitGutter See lines that have changed since previous commit in ST gutter.
GitSavvy Near perfect Git and GitHub integration.
Jedi Python snippets and completions.
Markdown Editing Powerful Markdown support with better syntax understanding and good color schemes.
open-url Open files, directories, URLs, and perform web searches for terms, from within ST.
PackageResourceViewer Extract packages into your Packages directory, so you can view and edit them.
Project Manager Without this package ST projects are basically useless.
Requester HTTP client built on top of popular Requests library.
Side Bar Provides enhancements to the operations on files and folders available from the side bar.
SublimeLinter Interactive code linting framework. (1)

(1) More on SublimeLinter.