If the amount of words you want to check doesn’t exceed 500, Quetext is definitely the best choice. However, the free trial will only allow you to check 500-word articles at a time. There is also a daily limit, although no one is sure if it's 3 articles or 5 (there are conflicting reports).
For larger texts, you can try Check-plagiarism.com. The upload limit is a generous 5,000 words, so it can work for essays and longer articles. But there is a catch with this one as well — it uses a sentence-by-sentence checking method, so you can’t know if your text as a whole plagiarises another.
As with most free tools, though, both of these can show false positives and have trouble identifying sources, which can make editing texts a nightmare.
My advice is to try these two out if you just have an article or two that you want to check. But if you have a larger word count and you really need the text to be 100% plagiarism-free, Copyscape is your safest bet.