Try Link-Vault. They're free and you benefit from the rating your site achieves based on traffic which they call "vaultage". As your vaultage increases, the number of sites your links are put on increases, as does the PR of the pages your links are allocated to.
The only drawback is that it requires some technical proficiency, as you need to put some PHP/ASP code on your site, but there is a users/members forum to guide you in difficulty.
If you decide to try it, please use my referral link - it won't cost you anything, but it will help my own vaultage; it works on a referral system, so you can bring in your own network and benefit similarly.
Your site requires pre-approval, but that's basically a formality, as long as it's not one of the usual questionables - gambling, porn etc.

My referral link is:
http://www.link-vault.com/?ss=4118

I'd really appreciate any referrals I could get if you or any of your pals decide to join.
Good luck