It depends too much on what good your site is worth.
Try to find a forum related to the subject and post it there, and if this is a universal solution, you can try a general forum like Something Awful.
In addition, you can create a Twitter account for the site and have your friends retweet beta invitation; what a quick way to get it to touch a lot of people if your site is interesting.
Sebastian Paaske TΓΈrholm
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