There are actually two Desktop GUIs for Counterparty: BoottleXCP and pyrpcwallet. pyrpcwallet is actually a lightweight wallet, so you don't have to download the entire blockchain.

What everyone is really excited for though is Counterwallet, a web wallet that looks really slick and will be available for testing this week. You can find screenshots here: https://www.dropbox.com/sh/738w1f3kgqik1za/ymr1VCWUv6 (and more info about Counterwallet here: https://forums.counterparty.co/index.php/topic,79.0.html.