Sometimes though you can appeal to a market you didn't realise was interested. The only freebies I like are those with tracking abilities.
Knowing your one site as I do, I can recommend a niche I'd almost be willing to bet you haven't tapped.
Marketing in general is a very difficult thing to do, as I've been looking at. I'm trying to get visitors to my blog, but its taking longer than I expected, at least to one of them. The other is increasing its popularity and I'm very happy with it, but I still have to find a way for the other one... we'll see. But since it is more of a personal thing, then I'm ok with going slowly!
Berlin,
If you want to post a link to the one that seems to be picking up slowly, maybe some of us could give you some advice on how to get traffic to pick up.