<?php $md5 = md5( strtolower(get_the_author_email()) );
$default = urlencode( get_bloginfo(template_directory) .'/icons/author.png' );
echo "<img alt='avatar' class='photo' title='photo' src='http://www.gravatar.com/avatar.php?gravatar_id=$md5&size=50&rating=X&default=$default'  />";
?>

Note that this assumes your theme has an author.png inside an icon folder. Change that line to reflect the location of your default icon, or use a blank default. If you're using Sandbox, or any other hCard-enabled wordpress theme, you can insert this inside <div class="vcard">, for added microformat-y goodness. Otherwise, you can put it anywhere you like. I've also added the same code to the top of my author.php file, inside <div class="archive-meta">.

I started with the 404 Search from Weblog Tools Collection, which simply redirects 404 pages to search results. That's not technically correct behaviour, so I borrowed the first few lines of my 404.php from their example:

<?php
$search_term = substr($_SERVER['REQUEST_URI'],1);
$search_term = urldecode(stripslashes($search_term));
$find = array ("'.html'", "'.+/'", "'[-/_]'") ;
$replace = " " ;
$search_term = trim(preg_replace ( $find , $replace , $search_term ));
$search_term_q = preg_replace('/ /', '%20', $search_term);
get_header() ?>

Then the rest of the 404 template, which you can check out here, follows normally from the sandbox file, until after the search field (which is important for usability). I don't use this in my index or single page templates, because 404's shouldn't land there (if they do, it's a problem with wp_query() ). After the search field, we jump into a custom loop:

<?php /* run the url as a query */
query_posts('s='. $search_term_q );
?>
<?php if ( have_posts() ) :
/* check to see if there are posts, before telling people something that isn't true */ ?>
<div class="content-column left"><h3>You might have been looking for these posts</h3>
<ul>
<?php /* start the loop */
while ( have_posts() ) : the_post(); ?>
<li><strong><a href="<?php the_permalink() ?>" title="<?php printf(__('Permalink to %s', 'sandbox'), wp_specialchars(get_the_title(), 1)) ?>" rel="bookmark"><?php the_title() ?></a></strong><br />
<abbr title="<?php the_time('Y-m-d\TH:i:sO'); ?>"><?php unset($previousday); printf(__('%1$s', 'sandbox'), the_date('', '', '', false), get_the_time()) ?></abbr><br />
<em>
<?php printf(__('Posted in %s', 'sandbox'), get_the_category_list(', ')) ?>
. &nbsp;
<?php comments_popup_link(__('Comments (0)', 'sandbox'), __('Comments (1)', 'sandbox'), __('Comments (%)', 'sandbox')) ?>
</em>
</li>
<?php /* end the loop */
endwhile; ?>
</ul></div>
<?php /* close the list before closing the ifelse */
else : endif; ?>

You can read the comments, but basically, we take the URL and use query_posts() to run it as a search, and return the results as a list, if there are any. So far, it works for posts that I've referenced elsewhere, but it's not returning the actual post itself. That's probably because I use category slugs in my URL's. and BTW, the fancy code highlighting in this post is courtesy of a fancy new plugin.

UPDATE: Some additional code was used from Arun's nearly perfect 404.

the latest undocumented features include atom API support. all that coding in app.php does little good if you don't reference it. going off of what my wordpress.com blog looks like, you should add this to the head of your blog:

<link rel="introspection" type="application/atomserv+xml" title="Atom api" href="<?php bloginfo('url') ?>/wp-app.php" />

you can implement that as a plugin, or in your theme. i'd love to see XML support moved to the core, but that might just be me.

when you're talking about meta tags, or feedburner RSS feeds, or custom title tags, you're talking about things you don't want to break when you change themes. and there's no reason for them to. this is how i do it: i wrote a plugin.

Gasp! Shock! horror!
not really. if you can handle everything in the post above, you can handle this plugin. here's how it starts:

<?php
/*
Plugin Name: archGFX Functions
Plugin URI: http://archgfx.net/
Description: all the things that i hack into themes, now they have their own file
Version: 1.0
Author: SunBurntKamel
Author URI: http://archgfx.net/
*/

you need to have a name, the rest of it's optional.

so, all that fun stuff that you want to add to the head of your blog? dump it into a function:

function archGFX_head() { ?>
<META NAME="Description" CONTENT="archGFX is the brand name used by Adam Freetly. This portfolio represents personal and collaborative work." />
<meta name="keywords" content="adam freetly, architecture, blog, portfolio, design, rendering" />
<META NAME="geo.position" CONTENT="41.92;-87.70" />
<META name="geo.region" content="US-IL" />
<META name="geo.placename" content="chicago" />
<link rel="shortcut icon" href="http://www.archgfx.net/images/blogrdfbg.jpg" />
<link rel="alternate" type="application/rss+xml" title="RSS 2.0" href="http://feeds.feedburner.com/archGFX" />
<link rel="search" href="http://archgfx.net/opensearchdescription.xml" type="application/opensearchdescription+xml" title="archGFX" />
<script src="http://www.google-analytics.com/urchin.js" type="text/javascript">
</script>
<script type="text/javascript">
_uacct = "UA-91088-3";
urchinTracker();
</script>
<?php }

obviously i have a few more things than average. google analytics scripts, geo tags, favicon, etc. you can call the function whatever you want, just remember it. now that we've got all our code, we need to insert it:

add_action('wp_head', 'archGFX_head');
?>

that's it, save the file, activate your 'plugin', and you're good to go. naturally, i do a few more things with this. i also handle my footer copyright notice the same way, with

add_action('wp_footer', 'archGFX_foot');

i add the mybloglog widget to my dashboard with
add_action('activity_box_end', 'archGFX_MBL');

and, i don't have to worry about forgetting to add any of it next time i switch themes.  the only thing that i don't put in the plugin is my title tags.  you can only have one of those.  so while i could add mine with a plugin, i'd have to delete the theme's title manually.

So, the plugin is nice, but you can also do it in themes, just add:

 <?php if ( is_category() || is_archive() || is_search() || is_author() ) { ?>

    <meta name="robots" content="noindex,follow"><?php

} ?>

<?php if (function_exists('is_tag')) { if ( is_tag() ) { ?>

    <meta name="robots" content="noindex,follow">

<?php } } ?>

to header.php. The UTW stuff (is_tag()) is a little annoying, but other than that, it's boilerplate.