Depending on how long ago that was, it's likely either BTC-e or Cryptsy. Have you tried checking your browser history?
Thanks for the answer RenegadeMind, it was in October 2012. I remember transferring it to a site that would allow me to convert the namecoin to bitcoin, but i never followed through.
As for checking history, I did the transfer on a computer that I've since formatted and re-installed the OS.