A couple of months ago I started looking for a plugin that would use my assigned tags as meta keywords on each post. After a brief search I came across MetaMagic, and I have been using it since.
You can also use the plugin to generate meta description tags, but you need to highlight a specific block of text on each post for this feature to work, so I find this part a bit inefficient. The good thing is that on the options page of the plugin you can choose what features to enable, so I just use it as a meta keywords generator.
AltimateTube 