Check out few Open source discussion forum and Stackoverflow style Q&A software
You can check out Discourse, OSQA, PhpBB, NodeBB
There is no general tools to use. First identity the project and contact the project owner, what tools to use and how to collaborate. Most of the open source projects use Git / SVN, Get familiarized with these will help to move forward.
Here are few links How to contribute to open source
Here are few open source projects came up while searching for Anti virus. http://www.findbestopensource.com/search/?query=anti+virus. Hope it helps.
ClamAV is the one which is popular and widely used. Most of the projects are either directly or indirectly using Clam engine...
I'm not quite sure how this site assigns scores to different projects. For that reason I don't think this necessarily defines the best client libraries. Take http://www.findbestopensource.com/product/gett-mongojs for instance. It has a two star rating but no justification to that rating or tangible review.
Check out these open source video platform projects. You can choose either Kaltura or Mediacore, both are equally good in managing videos and podcasts.
Here are few open source webmail projects. You can consider Mailpile, roundcube etc.
If you want to build your own email stack then consider Simple Mail Stack for Linux Hosted Environment
Here are few open source mail clients. You can choose Thunderbrid, Roundcube. If you are interested in web client then you can look in to MailPile.
From the link, I got information that, You want the following We need: - enter feature requests to different categories - add tags to the requests - let users vote for requests - sign up with social network accounts
You could try forum kind of software, where people could log a request, you could discuss and address. Below are few links, which may help you.
Here are few list of open source Text to speech software http://www.findbestopensource.com/tagged/text-to-speech. You could pick Mary, espeak, FreeTTS, Festival
You may need to learn a lot.. It is good not to reinvent the wheel, pick a programming language of your like [PHP / Java /AspNet] and choose a content mangement system in that language, Modify it to your requirement. You could build a site within few days. check out the free and open source content management system.!