These are my most used Ruby on Rails plugins:

active_merchant : e-commerce system
action_mailer_tls : Use Gmail’s SMTP
auto_complete
will_paginate
acts_as_tree
annotate_models : comment
restful_authentication : user authentication
acts_as_taggable_on_steroids
acts_as_versioned : ar-versioned
bundle_fu
exception_notification : sending email notifications when errors occur
render_component : call other actions for their rendered response.
calendardateselect
contacts
ssl_requirement
globalite : i18n/l10n
authgasm : Authlogic : authentication solution
acts_as_xapian
acts_as_rateable
rspec : Behaviour Driven Development framework
attachment_fu : file upload
sms_fu
rspec-rails : rspec_on_rails
dialog_helper
permalink_fu
engines : enhances Rails’ own plugin framework
acts_as_paranoid : ar-paranoid
cache_money : caching library for ActiveRecord
assert_select
factory_girl
guid : makes the “ID” field into a URL-safe GUID.
markaby
query_trace
simple_captcha
tzinfo_timezone : a replacement for the TimeZone class.
gruff : A library for making beautiful graphs.
localized_dates
routing-filter
ultrasphinx : Sphinx full text search engine.
state_machine : creating state machines for attributes.
shoulda : Making tests easy.
open_id_authentication
paperclip : file attachment library
upload_progress : track the uploaded status.
tiny_mce
responds_to_parent
testable-logger
rails-dev-boost : 10 times faster in development mode
redbox : lightbox library
headliner : DRYs up your page titles.
open_flash_chart
juggernaut
google_charts_on_rails : wrapper for the Charts API from Google.
asset_packager : CSS Asset Compression
calendar_helper
acts_as_nested_set
model_2_protobuf
rails-widgets : a collection of UI Widgets for RubyOnRails
unobtrusive
role_requirement
query_reviewer : advanced SQL query analyzer
mysql_index_prefix
clippy : Flash widget that place arbitrary text onto the client’s clipboard
active_scaffold
annotatedtimeline-for-rails : wraps up the Annotated Timeline widget
active_scaffold_export
strip_attributes : strips model attributes of leading and trailing whitespace
mongrel_proctitle
recaptcha
rake_tasks
custom_err_msg
action_mailer_layouts
active_record_base_without_table
acts_as_list
acts_as_rated
vote_fu
lipsiadmin
searchgasm : Searchlogic
acts_as_authenticated
authentication
mbleigh-subdomain-fu