BlockTerm lets you specify taxonomy terms to show this block on. When you visit a page where the node is tagged with one of these terms, the block will show, otherwise it won't.
Virtual Sites offers almost the same (and more) functionality as the Drupal multi-site feature without the need for the complicated setup of that feature. Depending on conditions (e.g. requested url or user role) handled by the Condition(s) module (bundled with Virtual Sites starting with the 7.x version), you can override theme, site information, menus and more to virtually present the visitor with a different website.
It is by no means limited to that, but one common example of where to use Virtual Sites is the mobile version of a site, intended to be viewed on mobile devices like smartphones or tablets. Using nothing else but Virtual Sites, you can set up all possible combinations of required functionality, like checking for:
a mobile browser (using the Browser condition);
a secondary hostname, eg. m.example.com (using the Hostname condition).
If any of the conditions above is met, you can:
switch the theme to a mobile-friendly one (using the VS Themes module);
even if the visitor used the regular URL, redirect them to the secondary hostname m.example.com (using the VS Common Settings module).
The Blog Reactions module monitors Technorati, Google Blogsearch, Yahoo! Bloglines and Blogato for reactions - trackbacks - on your posts. It displays a date and a short summary and links back to the original reaction on the web.