Plagiarisma (dot) net is one of the best free tools in have found so far. Forget about copyscape premium account.
Copyscape is really the most reliable unique articles checker. You can also try do it alone if you have time and don't want to buy a premium account at CopyScape.. Just copy one sentence from the article, put it in google "The sentence here" exactly as mentioned above and check if there are any results.. Check about 5,6 sentences from the article and once you see the results, you would understand if it is unique.. But I recommend using CopyScape
Copyscape is a really big one, but don't forget DupliChecker and Plagium. Those two are also good options.
Hi, pls have a try for smallseotools.com/plagiarism-checker/ & duplichecker.com/search.asp. I have been using them.
Thanks for your replies! My question wasn't accurate enough: I don't wanna search for copies of my sites. My idea is to use a free article spinner e.g. articlespinnertool.com and generate some articles with a uniqueness of above 80%. The problem is to determine the uniqueness of these generated articles. If you know a free tool where i can paste some articles and the tool checks the uniqueness for this set it would be great!