Most of them do actually turn out to be scammers (sooner or later) but so far, Hashnest has been somehow acceptable (although with a lot of issues, that you can read on their thread).
Website: https://www.hashnest.com/?locale=en
ANN thread: https://bitcointalk.org/index.php?topic=766448.0

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As much as possible, stay away from such websites and buy the mining rig instead (this way, you'll have full control over it [in case you've got a cheap electricity rate].)