Omeka is a digital platform designed by the Corporation for Digital Scholarship whom also developed the Zotero platform which helps to organize and share research, as well as develop bibliographies. While the organization utilizes the word corporation in their name, they are a not-for-profit company which began from the Roy Rosenzweig Center for History and New Media at George Mason University. They offer two versions of Omeka, a hosted(.net) and self-hosted version (software downloaded from Omeka.org). I have not had the opportunity to work with the downloaded version, but in reading through the website and looking at the plugins offered I do think the self-hosted version does have more flexibility in design.
The biggest benefit of Omeka.net is that you can begin working and publishing straightaway. They offer a free trial that allows 500MB of storage space and limited plug-ins, but that is enough to get started and get used to the interface. Omeka is best for projects encompassing lots of discreet items, but that does require lots of online storage space.
Omeka succeeds in providing an organizational structure for providing metadata for online archives. The established categories are numerous and broad enough to allow designers to provide robust information. The system also allows for multiple images for a single item which is helpful for presenting multipage documents or providing multiple angles or details of physical objects. 
While the collections feature might be helpful for some projects, I actually found it rather limiting as objects can only be associated with one collection at a time. I understand why the system is set up this way as it reflects the original method of organizing in an physical archive, but it does limit how the designer can relate items to each other. In order to re-order or re-associate items you have to create an exhibit which requires a lot more work than simply creating a collection, which is similar to creating a photo album on Facebook or Flickr.
So, skipping over the details of collections, I’ll move to building an exhibit. This part was not the most intuitive part of Omeka, nor does it provide much in the way of customization. With the paid versions, the user has more modules to choose from in creating the exhibits. The basic modules are text only, text with images, gallery, and file (I have a paid account and have access to image annotation, editorial, and location/map modules, although the image annotation block does not always load properly which is quite annoying). You can re-order the modules, but there is very little customization within each blocks. My biggest critique is the inability to easily change font size without creating bolded header-like text. The option open up the html coding exists, but I failed to get the text sizing code to work…but I have limited experience and was going off a random tutorial I googled.
Below is a video of my final project exhibit using Omeka to give an idea of how the exhibit modules work.
Traveling Medievalist 