For a very long time, I have been interested in a professional camera filter system. This is a one of the cheapest available on Amazon for just £12.99. It comes with many different ND filters and a protective pouch included - you'd want to use an ND filters for long exposures. This bundle of equipment also includes lots of adapter rings so the system can be used with most lenses. Also, the filters and holder are compatible with the more professional, more expensive Cokin-P filter system if you wish to upgrade in the future. I put the system to the test and found that light leaks and glare is a real problem. Also, trying to use with ultra wide angle lenses will be a waste of your time because of vignetting. Of course, the build quality could be better, but you really can't complain for the price.
Another downside to the filter system above is that it only includes ND filters up to 4 stops. I also bought a 10 stop filter by Formatt/Hitech for £28.68. It fit well in the holder and the sharpness was acceptable for a plastic filter, but glare and strange artefacts are a real problem with longer exposures. It also introduces a not so attractive colour cast, but this is easily corrected in photoshop when you're shooting in RAW. A bargain, but not the best.