Contesio

Bring a REAL search engine into your WordPress website in 5 minutes, for free!
Smart Search enhances the WordPress search engine by querying Microsoft BING and its Search API. 
The search query is performed in the context of your website url, just like you would type the following search query on BING:
<pre><code>site:http://www.yourblog.com find this and that in New York
</code></pre>
<h4>Brief</h4>
URLs from the BING result set are converted by Smart Search to WordPress post IDs, thus the display process is the same your theme implements for regular searches. 
Search results come from Microsoft BING. This means semantics and relevance on board!!
You can also choose to cache the search engine’s results for a defined period of time, which basically means that your site will load a little bit faster if the same search query is used.
In order to use it, you must have a valid <a href="https://datamarket.azure.com/account/keys" title="Create Windows Account key" rel="nofollow ugc">Windows Account key</a> and you will need to activate at least the <a href="https://datamarket.azure.com/dataset/8818F55E-2FE5-4CE3-A617-0B8BA8419F65" title="BING Search API | Web Results only plan" rel="nofollow ugc">free subscription here</a> or <a href="https://datamarket.azure.com/dataset/bing/search" title="Activate your Bing Search API plan" rel="nofollow ugc">here</a>
Easy, simple and fast! 
If this is your way and fits your needs, you could consider <a href="mailto:cristian@contesio.com" title="Get notified about Smart Search Pro" rel="nofollow ugc">Smart Search Pro</a> and its extended features.
Note: your blog content must be indexed by BING in order to see the matching result set. <a href="http://www.bing.com/toolbox/submit-site-url" title="Start indexing your site with BING" rel="nofollow ugc">Go here</a> to submit yuor site to BING. 
Feel free to leave a <a href="https://wordpress.org/support/view/plugin-reviews/smart-search" title="Review this plugin" rel="ugc">review here</a>. To get in touch with me you can <a href="mailto:cristian@contesio.com" title="Contact Smart Search developer" rel="nofollow ugc">drop me an email</a>.
<h4>Features</h4>
<ul>
<li>Works with built-in post types. Custom post types are supported as well</li>
<li>Highlight occurences in search results with custom background and text color</li>
<li>Auto detect misspelling and synonyms words</li>
<li>For post title and post excerpt, whether to display the WordPress or the BING one</li>
<li>Define how long to store search results for each query. This cuts down response time and saves transaction amount</li>
<li>Custom endpoint for empty search results</li>
<li>Use a custom domain as the search context, useful for testing purposes</li>
</ul>

Smart Search Engine

Smart Search replaces the native WordPress search engine by giving relevance to what matters. Results are displayed according to your current theme.

robfelty

Create collapsible archives by year or month. Features include: link to archive pages, display of individual posts and support for custom post-types.
<h3>Demo</h3>
I use this plugin in my blog at http://blog.robfelty.com

Collapsing Archives

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Smart Search Engine vs Collapsing Archives