In rebuttal to Geshan Manandhar's "must have" list, I propose these modules. I prefer to think of this more as a "must consider" list though.

1. FAQ - Many sites include a "Frequently Asked Questions" page; this module manages the list for you, so all you have to concentrate on is the content. If you want to add an "Ask the Expert" (or advice) "column." you can add the Faq_Ask module.
2. Weblinks - Almost every site has a "Links" page; this module manages that and allows you to have user-submitted links. It will even check the links on occasion to see if they still work.
3. Taxonomy Image - The taxonomy_image module allows site administrators to associate images with taxonomy terms, which can then be displayed in addition to, or instead of, the printed term.
4. Glossary - This module scans posts for glossary terms (including synonyms). The glossary indicator is inserted after every found term, or the term itself is turned into an indicator depending on the site settings. By hovering over the indicator, users may learn the definition of that term. Clicking the indicator leads the user to that term presented within the whole glossary or directly to the detailed description of the term, if available.
5. Site Documentation - This module display information from various places within the Drupal environment, saving you time searching through many pages. Some of the information comes from internal arrays, some is derived from system calls, and some comes directly from the database tables.
6. Event - This is a simple module to keep track of events and show them to users in various calendar displays. It also provides an event API to allow other modules to store and work with date data. There is an Event Repeat module for repeating events. It also works well with several event-related modules (e.g. Sign Up).
7. Mollom is a new spam fighting effort mainly initiated by Dries (Drupal Project lead) himself. With a central server to see if posts are spam or ham (not spam) it claims a high percent of success. You can get more information on its official website. This replaces the Spam module due to its long-delayed overhaul.
8. Image Attach - This module is part of the Image package; it allows you to easily attach image nodes to other node types.
9. Pathauto - This module automatically generates path aliases for various kinds of content (nodes, categories, users) without requiring the user to manually specify the path alias. This allows you to get aliases like /category/my-node-title.html instead of /node/123. The aliases are based upon a "pattern" system which the administrator can control.
10. XML Sitemap - This automatically creates a site map that conforms to the sitemaps.org specification. This helps search engines keep their search results up to date.
11. User registration notification - Notifies the administrator of new user registrations. You can even get emails when someone updates their profile as well.