How do I run only specific tests in Rspec?

You can run all tests that contain a specific string with --example (or -e) option:

rspec spec/models/user_spec.rb -e "User is admin"

I use that one the most.

Make sure RSpec is configured in your spec_helper.rb to pay attention to focus:

RSpec.configure do |config|
  config.filter_run focus: true
  config.run_all_when_everything_filtered = true
end

Then in your specs, add focus: true as an argument:

it 'can do so and so', focus: true do
  # This is the only test that will run
end

You can also focus tests by changing it to fit (or exclude tests with xit), like so:

fit 'can do so and so' do
  # This is the only test that will run
end

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.