But I have to ask this anyway, I had a question other day "Which open-source download manger project is now popular/on the rise/looks promising?" Because I wanted to start contributing, then I asked myself where should I ask this so I get the most correct answer? But opensource.stackexchange does not allow this type of question, so where should I ask these type of questions?
First off, thank you for asking us this on Meta instead of just going ahead and asking the question :)
You've got a few options.
If you want to stay on Stack Exchange, then there's potential to ask your question on Software Recommendations. Be aware, however, that they require questions to adhere to some fairly strict quality requirements - you should read those first. You will need to specify exactly what you want this download manager to have or to be - open source can be one of those criteria, and you can have something about active users or contribution activity, but you'll probably need other details too.
Reddit's /r/Software deals somewhat in recommending software, and their quality requirements are slightly more lenient. I'm less familiar with the system there, but you should be able to find some way of asking your question there.
Similarly, Reddit has /r/opensource. I'm not sure whether they deal with recommendations or not, so you'd have to find their guidance or ask a moderator there. If they do, you may be able to ask the question there.
I hope that gives you some options.