At Shapeshift you can easily and (rather) quickly exchange (or “shift”) inbetween a selection of the most popular cryptocurrencies, without creating an account. You need a wallet to receive your “shifted” coins, then just send BTC (or the coin you are interested in exchanging) to Shapeshift’s web page. You may input a return address too, in case something goes wrong, but it’s optional.
As Shapeshift doesn’t handle any fiat monies (USD, euro etc) they don’t have to follow KYC rules, so there’s no need for entering any personal info (which speeds things up). The site’s FAQ warns against using it to “tumble” though (to “clean”). The service shouldn’t be confused with so-called “tumblers” or “mixers”, designed to obfuscate the original source (address).
“Shifting” isn’t instantaneous though, due to a wait for a Blockchain confirmation (according to the site’s FAQ). There is also a deposit limit (4.5 BTC, when publishing this post).
The site’s support staff is very attentive and responds quickly.
There is also an app (an iPhone-version of Shapeshift) available.