Besides those already mentioned here, I also found the following necessary:
- Announcer - very flexible notification scheme
- AutocompleteUsers - handy while typing (existent) user name
- AutoLinks - automatically make words not conforming to wiki naming rule but matches existent page name a link
- CustomFieldAdmin - make manage custom fields easier
- Redirect - handy if you constantly need to make short-hand name wiki pages (like HTML redirects to HyperText .....)
- TicketDelete - make deleting, if at all needed, easier
- WikiRename - must-have for wiki refactoring
Below are good-to-have:
- S5 - directly render wiki pages as slideshow in S5 format, could be really useful for using Trac as the source for presentation
- FullBlog - add blogging support to Trac
- Vote - cool add-on feature for big team
- TracWikiToPdf - transform wiki page to pdf dynamically (however the effect might be all that satisfying)
- TimingAndEstimation - neat for tracking time and/or estimation