"must have" modules for a blog

It depends on what you're looking for, but the short list of contributed modules that might be helpful to you are:

• Blog Info adds a block to your weblog pages with space for your bio, picture, etc.
• CAPTCHA helps prevent spam comments, and reCAPTCHA may be the coolest captcha around at the moment.
• Coral Defender prevents your site from getting slashdotted, fireballed, etc. by redirecting requests to the Coral Cache.
• Node Clone makes it easy to create another post based on another, existing post.
• Fast toggle adds node links to the bottom of your posts letting you toggle comments, published state, promoted to front page, etc. for individual posts.
• Global Redirect protects your site from getting sandboxed by search engines by redirecting from the original node path (e.g. node/1) to the aliased path (e.g. about/colophon).
• HTML Corrector closes HTML tags and can prevent runaway, open tags from styling everything after it (in comments, for example). HTML Correcter is part of core in Drupal 6.
• Path auto can automatically create paths for your posts, which creates a consistent, human-readable website structure.
• Remember Filter will keep track of which input format you use for certain node types. There's a patch in the issue queue that updates Remember Filter to work with D5.

Modules that may be helpful to your visitors are:

• Accents extends the core search module and makes it easier to search for words with accented characters.
• Alinks turns certain words and acronyms in your posts into links.
• Archive generates a reverse chronological listing of your posts and offers yearly, monthly and daily views.
• Comment Subjects can autofill the "Subject" field for comments (if you have that textfield enabled).
• Comment Info allows anonymous visitors who leave comments to enter their contact information once and have it autofilled in the comment form for the next time. Part of core in Drupal 6.
• Search 404 offers search results for keywords in the URL in place of the standard 404 page not found error.
• Porter-Stemmer extends the core search module and improves search results by searching for the root of search keywords.
• Technorati submits your weblog to Technorati's weblog indexer and can tag your posts in several useful ways.
• Service Links provides links (text, icons or both) to submit your post to websites like Delicious and Digg and can display those links in a block or in the node links at the bottom of your posts.
• SpamSpan wraps email address in a JavaScript to prevent email harvesters from collecting email addresses in your web pages. It degrades gracefully when JavaScript is turned off.
• Tagadelic generates tag clouds and gives your visitors an easy way to see what's available on your website. It creates the tag cloud from terms in a vocabulary you determine.