To avoid bots and spammers, I recently changed the visual CAPTCHA for a smarter one, requiring from the person registering to the forum to sort a few virtual world names in two lists: the first is the list of virtual worlds that the Cool VL Viewer can connect to, the second is the list of virtual worlds you can't connect to with the viewer.
Everyone knowing what this forum topic is all about should be able to pass this CAPTCHA and it proved 100% reliable against bots and spammers so far.
To move a name from one list to the other, simply left-click on it with the mouse pointer and, keeping the mouse button down, drag the name from one list to the other, then "drop" it by releasing the mouse button.
Here is what the list looks like prior to sorting (simply move the names of the virtual worlds you can't connect to with the viewer from the top list to the bottom one):

If Javascript is not enabled in your browser, you will get two lists with check boxes instead:
