What alternatives to IRB are there?

Use Pry: https://github.com/pry/pry

Let's you:

• start sessions at runtime
• view method source code
• view method documentation (not using RI so you dont have to pre-generate it)
• pop in and out of different contexts
• syntax highlighting
• gist integration
• view and replay history
• open editors to edit method using edit-method obj.my_method syntax

A tonne more great and original features

What a coincidence. Rubyflow just yesterday announced the irbtools gem, which is a meta-gem containing lots of cool irb enhancement gems. It contains:

• Colorized and output as comment by wirb and fancy_irb
• Nice IRB prompt and IRB’s auto indention
• Includes stdlib’s FileUtils: ls, cd, pwd, ln_s, rm, mkdir, touch, cat
• Many debugging helpers: ap, q, o, c, y, Object#m, Object#d
  • ap – awesome_print
  • q – like p, but on one line
  • Object#m – ordered method list (takes integer parameter: level of nesting)
  • Object#d – puts the object, returns self (using tap)
• “Magical” information constants: Info, OS, RubyVersion, RubyEngine
  • OS.windows?
  • RubyEngine.jruby?
  • RubyVersion.is.at_least? 1.9
• Clipboard features: copy and paste
  • also available: copy_input and copy_output for session history
• Call vim (or another supported editor) to edit a file, close it and it gets loaded into your current irb session, powered by interactive_editor
• Another way of live loading into irb: sketches
• Highlight a string with olorize('string') or a file with ray('path'), powered by coderay
• Displays ActiveRecord database entries as tables with hirb
• Restart irb with reset! or change the Ruby version with the use method and rvm!
• Includes the current directory in the load path (was removed in 1.9.2 for security reasons, but is pretty annoying in IRB)
• Shorter requiring like this: rq:mathn
• And rerquiring with rrq
• Try the included Object#ri helper, powered by ori!
• Access to a lot of more commands with boson – call commands to get started

There are nice screenshots on the irbtools page. One nice thing about it is that each of the utilities can stand on its own, in case you just want to cherry-pick one or two features.

2013 Update

Since I wrote this, Pry has become a popular IRB replacement. It doesn't do as much as irbtools out of the box, but it extensible with plugin gems that add cool features. You can browse source code like it was a unix directory:

pry(main)> cd FileUtilspry(FileUtils):1> show-method rmFrom: /opt/ruby/lib/ruby/1.9.1/fileutils.rb @ line 556:Number of lines: 10Owner: FileUtilsdef rm(list, options = {})  fu_check_options options, OPT_TABLE['rm']  list = fu_list(list)  fu_output_message "rm#{options[:force] ? ' -f' : ''} #{list.join ' '}" if options[:verbose]  return if options[:noop]  list.each do |path|    remove_file path, options[:force]  endendpry(FileUtils):2>

You can also browse Ruby documentation, issue shell commands, and if you're a Rails user, you can use the pry-rails gem to get pry in your Rails console. There's also a way to hook it into Sinatra and use it with Heroku.

There's ample documentation--there are a bunch of screencasts including a Railscast. It's definitely worth looking into.

I've never heard of a (popular) alternative to IRB, but there certainly are several useful gems that make the IRB experience a lot nicer:

• awesome_print pretty prints Ruby objects with indentation and coloring, very useful when trying to look at nested hashes or other complicated data structures.
• looksee is pretty awesome too, it provides a method lp (lookup path) that shows you where a Ruby object gets its methods from (class, superclass etc).
• Sketches connects your editor and IRB, so it's especially useful if you are the type who likes interactive development. Emacs with inf-ruby is also good for this.
• Wirble is a whole set of IRB enhancements, like tab completion and syntax highlighting. There's also Utility Belt, but I don't personally use that, so can't comment on its features.

Edit

I forgot Hirb, which is very useful for e.g. showing the results of an ActiveRecord query in a Rails console.