Search is ubiquitious. It's available on all sites, desktop applications, ... A good search engine is something essential for letting your users get what they want. There's a lot of factors that define what a makes a good search engine: speed, accuracy of the results, ...
Drupal has already a plethora of solutions these problems. There is a search module in core. We also have integration with Google Search Appliance, Custom search, Lucene, ... But most recently Apache Solr is hotter than hot and seems to become the standard replacement for Drupal's core search solution. Even more now since Acquia uses it as the core of one of its flag products, Acquia Search.
I just released a new Drupal module. It was one that has been laying around for some time now, but got constantly improved to make it reusable on as much sites as possible. I stripped the feature set down to the minimum and called it "subsites".
Download it at its Drupal project page.
Subsites are a part of your website that can have its own menu, theme, custom CSS or anything else you want.
An example? Suppose you are making a magazine website. On most magazine websites you might want to create small subsites about a certain topic. This subsite probably has its own menu (next to the primary navigation). It might also have its own look. This new look could be a minimal change from the default site look (a different background, different colors, ...) or it might be a completely new look.
Some time ago I released the Campaign Monitor Advanced module. A module that provides advanced integration with the excellent Campaign Monitor mailing list service. Cool stuff was available in it like support for multiple lists, automatic fetching of lists and fields from your Campaign Monitor account, asynchronous pushing of subscription and unsubscription requests and the ability to match Campaign Monitor custom fields to Drupal tokens, profile fields and PHP code. Cool huh?
Well, you'll be happy to know that all this stuff is available for more services then Campaign Monitor only. I have completely rewritten the module as a framework, called E-mail Marketing Framework, to make it work with virtually every mailing list service that provides an API to some of its functionality. Right now, out the box only MailChimp and Campaign Monitor integration is provided.
Thanks to the the sponsorship of Drupal integrator Calibrate, I've just released a new Drupal module that integrates your Drupal site with your Campaign Monitor account.
Campaign Monitor is a fully rebrandable online mailing list tool, and by adding this module you can integrate the users of your Drupal site with your Campaign Monitor subscriber lists, in a quite advanced way. This module includes features like:
Visit the Drupal site to grab the Campaign Monitor Advanced module. If you don't need that level of integration, you can use the simpler existing Campaign Monitor module, which is still available.

I just contributed a new Drupal module called Service Attachments. It basically finds related content for nodes on others sites like YouTube, Flickr, Twitter, ... It's actually implemented as a small "framework" so you can plugin in your own services.
When do you need this? Well, when you want to enrich your nodes with content from other sites and you don't want to go through the process of searching all those sites.