Here are some "frequently used" modules

SEO (Search Engine Optimization)

Path
/node/1 -> /welcome
Pathauto
Automatically sets a path (based on title, classification, etc.)
Should use with taxonomy
Nodewords (meta tags)
Meta tag system to insert tags ("set and forget")
XML Sitemap (gsitemap)
Takes content and organizes with updates (autopilot)
Good theme...
not a module, but a good theme, puts data first

Content

Taxonomy
classification of nodes... NOT "category"
Views
Slice and dice content
Panels
Plays well with views
Diff
Compares new and old versions
CCK
Create custom content types, plays well with views
Can be used to create custom forms

Administration

Update status
Tells when modules are updated
Login tobaggon
e-mail activation, and signup

Community

Forum
Comments
Akismet or captcha
captcha can use textimage (accessibility)
Organic Groups
Organize people
Buddylist
Keep track of your buddies
Private MSG
Send messages
serviceLinks
digg it

Development

Devel
Gives a lot of development tools, mashed together for convenience
Don't run on a production site

e-Commerce

eCommerce
Sell stuff... maybe wait for uberCard
Requires token

Editing

tinymce
Best of a terrible thing
OR QuickTags
Can't use with tinymce

Images

image
imagefile
imageassist
Private
No