Team Web Access (TWA) has a very powerful feature: it allows to search workitems using free-text queries. It also allows to narrow your search by some metadata (similar to SharePoint search).
Example: the following query will return all bugs containing word "navigation"
navigation T:Bug
Unfortunately TWA runs a free-text query only against a limited number of properties (Title and Description if I not mistaken). It doesn't include History field, and the worst thing is that this set of properties cannot be configured.
I suggest to implement similar feature in Fissum, but with ability to configure set of properties included into free-text search. I can take care of development if you add me to the project.
