Sathya's answer focuses on topicality. Questions can also be on-topic but be a bad fit for Super User for other reasons. Brainstorming questions often aren't a good fit because the question seeks to compile a list of ideas, and may not have an objectively right or best answer. Similarly, troubleshooting questions can't be handled well in a Q&A format.
These kinds of questions can often be handled better in an open-ended discussion format as found on a forum. Some forum recommendations:
Windows
- WindowsForum
- Windows "N" Forums
Linux
Product-Specific
Most product developers have their own support web site, with a concentration of people very knowledgeable about their products. This can generally be found by going to the product web site and looking for a support link. A few umbrella organization support sites: