Tobu is an information manager written in Python that relies on columns, very much like e-mail programs, which in turn seem to be indebted to spreadsheets. It has therefore some similarities with Agenda.
Its main constituent are titles, tags and views. You enter items by name and then add tags. The tags are shown in the view as columns. some tags are special, they result in sortable columns. The tags for sortable columns are written like this "cost: 100" or "chapter: 01", and the value after the colon is used to sort the view. "When Tobu sees a sortable tag, it inserts the value of the tag in the column under that tag; when we're dealing with regular tags, an 'x' is inserted to indicate that the tag is present in the record, an empty space indicates that it's not present. You can select the columns you want to see from the top and the program will show just the items who possess the tags you have typed into the box." Tags are sorted alphabetical or numerical, but there "are a few special values that will be sorted in a different way: short month names, short day names and priority descriptions - 'low', 'medium', 'high', 'urgent'".
Titles are like items in Agenda, tags like categories, views like views. Clicking on a column head of a sortable tag will order the view in accordance with this tag. Views in Tobu are really just filters. This is probably true of Agenda as well. However, the tags constitute a flat list, not a hierarchy as in Agenda. And there is no automatic assignment of categories. Nor completion proposals.
The grammar of sortable tags is not that different from attributes or properties in ConnectedText.
The claim is: As the number of items grows to 5k, 10k, 50k and more, it becomes exponentially harder to find a set of relevant items; it's also harder to enter items in the tree structure because you have to choose the most important category among many possible ones. Full-text search helps but it will often produce a set of results that is too big or too small - it is particularly difficult to use search if you're looking for a set of items instead of just one item, and you won't be able to sort results as you may do with Tobu.
It's an interesting project that has possibilities which for the most part are unrealized yet. At this point, I find its concept more important than its usability.
It's donation ware and it works on Linux as well.