Plugins and Gems for Ruby on Rails that I've known and loved

There are a bajillion “best of” lists for Rails gems and plugins out there. I thought I’d contribute one more with some less well-known gems and plugins that I rely upon every day.

• Haml - A terse, elegant view-level syntax that replaces icky, old ERB. It took me about 20 minutes to get used to Haml, and now I'll never go back.
• Thinking Sphinx - Need to do full text searches on your MySQL database? Then you need Thinking Sphinx. TS definitely has some rough edges, but these are more than made up for by its flexibility and incredibly responsive author.
• Formtastic - I'm just starting to get a real feel for Justin French's Formtastic gem, but what I've seen so far makes me incredibly happy. Formtastic takes the drudge work out of creating forms, so you can get back to more interesting work.
• friendly_id - The purported "swiss army bulldozer" of slugging and permalinking solutions for Rails. What really struck me about friendly_id was how easy it was to set up and get running. No fuss, no muss. Just slugs and great-looking permalinks.
• Currency - A few weeks ago I was hunting around for a piece of Ruby code that could convert arbitrary representations of money (e.g. "$10", "10", "$10.04", etc.) into my preferred database format, an integer representing number of cents (e.g. $75 == 7500). I finally found Currency, which made this a braindead simple task. Whew.
• Jrails - Drop-in Jquery replacements for the Prototype/Scriptaculous stuff in Rails? Check, double check and AWESOME.
• Timed Fragment Cache - Automatic expiration of cached page fragments without any muss or fuss, plus proper compatibility with Rails 2.3.

What are your favorites that no one else knows about?