The site logo variable is used by many contrib modules but the way it is rendered by default leaves little room for customization. Usually it is rendered by the theme in a pre-defined region, linking to the home page and displaying the full image without any possibility of using image styles. Logo Block solves these problems by creating a block that you can put wherever you want, with a configuration form where you can select which page (if any) you want the image to link to, as well as an optional image style that you want to use for the rendering of the logo.
Configure the block at yoursite.com/admin/structure/block/manage/logo_block/logo/configure. Follow the instructions for the link field to optionally add a link and an image style for the logo.
Note: Due to limitations in Image API, image styles only works for logos uploaded to the Drupal files directory or subfolder.
Blockify exposes a number of core Drupal elements as blocks, including the site logo. However, it doesn't allow for the kind of customizations that Logo Block does.
Delta contains a submodule called Delta Blocks, which has no dependencies and offers similar functionality as Blockify except it works better and is more flexible. Again, however, it is not as flexible as Logo Block as it does not allow for custom url linking or image styles.
This module is sponsored by Storleden. Creator and current maintainer is Kristofer Tengstrom.