Reviews and Ratings
I've been using Flyspray for several years so far and have found nothing better.
Flyspray provides everything I need, starting from a powerful ticket system with the support for a wide range of parameters, to the easy-in-use control panel. It can handle multiple projects in the same system, each with its own custom settings and parameter types, such as ticket type and category. I especially enjoyed the permission system, with the support for anonymous tickets and per-project settings. Another advantage is the user interface that is elegant and functional.
The only thing I miss is the text formatting system. The tickets are displayed in the plain text, whereas I would like to use some form of formatting (even HTML). However, it is not a very big disadvantage, as the plain text can be also well-formatted and clean.
MediaWiki is undoubtedly the most advanced wiki engine that has been already written; at least I have not found anything similar. It is the best for creating a knowledge base with Wikipedia-like structure and organizing the community content, for example tutorials and articles.
One of the advantages of Mediawiki is very rich set of community extensions that add extra features to MW. Many of them have a very good quality, especially if they are used at Wikipedia. At the second place, I would put the powerful text formatting syntax.
The only quite significant disadvantage is quite ugly source code. I find it annoying especially if I need to customize the default layout for my own needs. Fortunately, many changes can be done directly through editing the special wiki pages that represent CSS styles, system messages etc. - we just have to know, what to change to achieve the requested result.