My Must Haves:

Ventrian Subscription
DNNCreative Subscription
PageBlaster
Mitch Sellers' Expandable HTML/Text
A CSS menu, House Menu, Snapsis, etc. OR (Prefer) Telerik RAD Controls

Site Dependent:

XMod
Active Forums
Ultra Media Gallery
Bring2Mind's Document Exchange
DataSprings' suite
OnyakTech suite
iFinity URLMaster
Catalook

I don't, or at least haven't yet, work on social networking sites so I don't have any modules in that field that I use.  Others might.  For some clients I might use just one or two of the DataSprings or Onyaktech products but I prefer to license the suite.  If they had developer licenses like Telerik or more liberal use licenses like Ventrian I would probably use them more.

Then again, what I consider must-have items a client can always override...  :)

Jeff

Another must-have: 
My list of "must-haves" would definitely include one of the free "SQL" modules such as AdvancedDatagrid (ADG) or SQLView.  These are utlitily modules that enable me to fill in many "cracks" without having to spin my own custom modules for simple tasks.  I most often use them to extend functionality of existing modules but I've also used them to create standalone modules in their own right. 

I've blogged about ADG (with some examples) here and about SQLView (also with an example) here.

 
Expandable Text/HTML
Read about (and download) the expandable HTML/Text module here:
 
  http://www.iowacomputergurus.com/free-products/dotnetnuke-modules/dnn-expandable-text-html.aspx

...and the forum page for the module:
 
   http://www.mitchelsellers.com/forums/view/topics/forumid/18.aspx

 
-mamlin