How do I run only specific tests in Rspec?

You can tag examples with :focus hash attribute. For example,

# spec/foo_spec.rb
RSpec.describe Foo do
  it 'is never executed' do
    raise "never reached"
  end

  it 'runs this spec', focus: true do
    expect(1).to eq(1)
  end
end

rspec --tag focus spec/foo_spec.rb

More info on GitHub. (anyone with a better link, please advise)

(update)

RSpec is now superbly documented on relishapp.com. See the –tag option section for details.

As of v2.6 this kind of tag can be expressed even more simply by including the configuration option treat_symbols_as_metadata_keys_with_true_values, which allows you to do:

describe "Awesome feature", :awesome do

where :awesome is treated as if it were :awesome => true.

Also, see this answer for how to configure RSpec to automatically run ‘focused’ tests. This works especially well with Guard.