Back at the start of the year when I was building a website for some self-catering cottages on the Isle of Man, I needed to replace some availability calendar functionality that I'd originally coded for the site back in 2000 or so.
As I was moving the site to Drupal, the availability calendar solution really needed to be something that was fully integrated as a Drupal module. I had checked out a couple of modules and the two that I found (Availability and OpenResort) didn't really do what I was looking for, so I went about customising the Availability module to do what I wanted.
Soon enough, the module had changed shape quite considerably, so it didn't feel like it would fit back into the original module. After about six months of running the module, the new Availability Calendars module got released back to the community last night.
If you run a small accommodation provider business (like a self-catering cottage, a bed and breakfast, a guesthouse, etc.) and are using Drupal to power your site, you might find this module quite useful. It will allow you to show an availability calendar for each node of a certain type (e.g. cottages, rooms, etc.), change the availability for each day, and also leave a note (such as a rental price) next to each week.
If you have any issues with the module or would like to request new features, please add a comment to the issue queue.