OMG, this is an amazing Rails plugin : CookieStore - This lets you store all your session data in a cookie on the users browser.

Now, if you've designed your application right, and don't store tons of data in the session, this looks like a great solution to avoid having to use memcached for sessions, or active_record_store.

If, on the other hand, you do put a lot of data in your session, it might not be that great for two reasons. First, you're restricted to storing 4kb or less of data, and Second, unless you use the encrypted version, a hacker could manipulate the session data in the key.

Still, an amazing looking plugin.