I, like many others would like to have captcha enabled, well while in the support forum this was neglected, I found my own workaround and want to share it with the community here.
In my case I need a captcha for the upload section to stop bots driving traffic to my server.
Here is what I did:
ATTENTION advanced users only! Don't do it if you do not fully understand what is happening!!!
1. I found some 3th party plugin called OSOLCaptcha -> Install -> enable follow the instructions etc.
2. navigate to components/com_jdownloads/jdownloads.html.php
find line containing: " $form = 'triggerEvent('onShowOSOLCaptcha', array(false));
start line again " $form .='"
3. Go to your front-end page -> refresh -> Now you should see the captcha! WARNING IT IS NOT working yet.
4. Front-end page -> HTML Source Code -> Copy the captcha related HTML, it is wrapped within a div tag " ... "
5. Go back to jdownloads.html.php -> restore it like it was in original.
6. Again in jdownloads.html.php locate the " it to appear.
Remember that it has to be IN THIS form tag to work properly.
By the time of this writing I am using jDownloads 1.7.5 and OSOLCaptcha 1.0.6!
This is a core hack and you must have this in mind when updating jDownloads.
I hope I've helped you all a little.