List of top 200 Drupal Module that you should know...

accordion_blocks
active_tags
ad
addtoany
admin_menu
advanced_help
advuser
apachesolr
archive_by_terms
audio
autoassignrole
avatarcrop
backup_migrate
bcs_attendance
bcs_leave
bcs_timesheet
bcsi
beautytips
bitcache
calendar
captcha
cck
chatroom
colorpicker
comment_upload
community_tags
contact_importer
contemplate
content_access
content_profile
content_slider
counter
ctools
date
dbscripts
ddblock
dhtml_menu
directory
ds
easySlider
email
event
faq
fckeditor
feedapi
feedapi_mapper
feedback
filefield
fileframework
fivestar
flag
flashvideo
flvmediaplayer
formfilter
forward
friend
friendlist
gallery
gamabhana_drupal
getid3
globalredirect
gmap
google_analytics
guestbook
heartbeat
hierarchical_select
homebox
htmlmail
i18n
iframe
image
image_title
imageapi
imagecache
imagefield
imagemenu
imagex
imce
indic_script
inline
interests
interface_sortable
invite
jcarousel
jcarousel_block
jcarousellite
job_queue
jquery_countdown
jquery_drag_drop
jquery_plugin
jquery_ui
jquery_update
jtooltips
kudos
lang_dropdown
languageicons
lightbox2
link
lm_paypal
location
logintoboggan
mail_api
messaging
mrbs
multiselect
multisite_info
nice_menus
node_clone
node_gallery
node_import
node_type_filter
nodeaccess
nodeasblock
nodewords
notice
og
onepageprofile
page_title
page_view_pager
panels
pathauto
phpfreechat
plus1
poormanscron
popups
print
privatemsg
profile_privacy
quick_informer
quicktabs
rdf
recipe
registrar_api
robotstxt
rules
scheduler
scrollable
securesite
seo_checklist
seo_friend
service_links
services
signup
simple_paypal
simple_reservation
simplenews
site_map
skinr
slider
slider_textfield
slideshow_creator
sn_extend
stringoverrides
subscriptions
switchtheme
tagadelic
taxonomy_browser
taxonomy_image
taxonomy_manager
taxonomy_menu
taxonomycloud
terms_of_use
text_resize
theme_editor
theme_generator
tinymce
token
translation_overview
twitter
ubercart
uc_product_power_tools
user_activity
user_register_notify
user_relationships
user_stats
user_visits
username_check
userpoints
userpoints_top_contributors
userpoints_votingapi
userreview
vertical_tabs
video
video_filter
views
views_accordion
views_calc
views_rotator
views_showcase
views_ticker
viewscarousel
vocabindex
votingapi
webform
webformblock
webmail_plus
wysiwyg
xmlsitemap