If you're using ImageField with CCK, then you should be using this module.

You can define an unlimited number of preset image manipulations that can be called upon when displaying images. These "manipulations are usually just different sizes, but the module also give you the option to crop, rotate, desaturate, and sharpen.

What's with the "Cache" in ImageCache? It actually only creates a modified image when that image is needed. Once a modified image is created, it is cached and treated just like any other image on the server. The next time that particular image modification is called, it served from the cache - no additional image processing necessary.

ImageCache also integrates fully with Views to allow you to select how an image appears in a view.

Stats

• Learning curve: medium - the underlying concept behind ImageCache takes a few minutes to grasp, but the actual implementation is quite simple.
• Depth: deep - killer intergation with CCK and Views and intelligent caching has encouraged a bevy of modules to rely on ImageCache.
• Resources:
  • Official Drupal.org handbook page
  • Download this module from the official Drupal.org project page

This module integrates well with various other Drupal modules including Views, Actions (part of Drupal core), Rules, and Token. It also has an API with limited documentation.

Flags can be global or per-user. Global flags are shared among all users - an example might be "flag as offensive". Per-user flags are not shared - an example might be a user "favorites list", where they flag their favorite content on the site and then a list of all their "favorites" is placed in a sidebar.

The Flag Weights module makes it possible to reorder flag in an arbitrary manner.

Stats

• Learning curve: low
• Depth: fairly deep, the great integration with Views really makes this module powerful
• Resources:
  • DrupalEasy articles on this module
  • Official Drupal.org handbook page
  • Download this module from the official Drupal.org project page

Views is a universal API to let any user or module make a list of just about anything on your Drupal site: views comes with the ability to list all of Drupal's "first class objects":

Nodes

most views list nodes, and nodes have the most power and felxibility in a Drupal site

Aggregator Items

list stories in an RSS feed, sort them, filter them

Comments

maybe list all comments in moderation, or comments from users with the Editor role

Files

think: all files by one user, all MP3 files

Node revisions

Peek into the history of your content

Taxonomy Terms

Create custom navigation fast

Users

Combined with something like Userpoints, a list of folks who are active on your site

Access Log entries

find out where users are hitting your site, how they got there, and when

Stats

• Learning curve: steep
• Depth: incredible, especially once you consider relationships, attachments, AJAX and theme information
• Breadth: tons of add-on modules, tons of modules use views internally or add features to this module
• See also: simpleviews
• Resources:
  • DrupalEasy's articles about Views
  • Screencasts
  • Download this module from the official Drupal.org project page