Few days ago, I saw some sites (forgot the URL's) where you give a fiction or non-fiction piece, if they publish it, the copyright rests with them for 6 months (limited time) and then revert it back to the author. I have seen this in very few sites, could anybody enlighten me or share any web-sites where they have seen it like that.
Since @shirish and I had quite a back and forth in chat, I'm going to summarize the discussion.
The OP does refer to sites where you submit your work and publication is determined by an editorial staff. It does not refer to sites where you just post your writing.
I recommended he avoid sites which either ask for all rights that they then return to you after some given amount of time or want to enforce a Creative Commons license on your writing. IMO, most reputable journals will ask for first time rights of some sort, with rights reverting to the author upon publication. Many will ask for the right to reprint. Also, many will include the information on rights on their website under Submissions or Guidelines. For an example, see http://www.bellaonline.com/review/submissions/ Mused is an online journal which has published several of my poems and has a Rights section clearly given on their Submission page.
There is a difference between copyright and the rights assigned to a publication. The author has the copyright but assigns certain rights to the publication. In the example in the link above, they ask for first time worldwide rights, which revert to the author upon publication.
I believe there are many journals out there which are clear about this kind of information. Rather than attempt to provide a list, which is both off-topic and a hefty job, I made two recommendations. One is duotrope.com, which has an extensive database of writing markets. The other is Writer's Digest's marketing guides. When cost is a factor, it is sometimes possible to find the marketing guides at a library. With both Duotrope and the guides, I recommend going to the journal's website to view their current guidelines.