I created a bookmarklet for this purpose. You can get it at the URL below. Just drag the link to your bookmarks bar and then click it. It will create a link above each table to allow you to download that table as a CSV. One caveat is that it won't work in IE as IE won't allow data uri's for anything other than images ... haven't tried it on Microsoft Edge yet.
It's worth noting that since I posted this answer there is now a free add-on in the marketplace for the server edition that will allow you to download a table as CSV. Link is below. Get version 1.1.3 of the add-on if you are on Confluence 5.x and the newest if you are on Confluence 6.x.
I don't know of an easy way. There are two ways that do work though.
The first one saves a step, but it can be VERY tricky to select all the text in a Confluence table.
You should try both methods as the resulting text style is different.
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