One of the most popular Android custom ROMs is the one created by the Paranoid Android team and now they have managed to integrate the Android L-style UI in their latest PA 4.5 Alpha 1 builds.
If you're a fan of custom ROMs and you like to tinker with your device's ROM then you should try and use the Paranoid Android 4.5 Alpha 1 ROM. It comes with the first preview of Android L-style Recent apps screen that is implemented from Google's future Android version. As you already know Google is planning on launching the final Android L (5.0?) build in autumn, but it has already made available the Android L Developer Preview system images, so that developers will start adding Material Design UI elements in their works.
Since the PA 4.5 Alpha 1 ROM is not a final build you might find errors and bugs, it would be best for you to avoid installing this ROM for a daily basis usage. In case you're a fan of stable ROMs then it would be best for you to stay away from these alpha builds of Paranoid Android.
In Paranoid Android 4.5 Alpha 1 ROM build you will find ParanoidOTA Fixes and cleanup, Fix for daydream duplicates, Theme Engine upstream patches, QuickSettings bug fixes, PIE will now be activated automatically for apps that are using immersive mode. The Recents CardStackView Redesign Preview is available only in the Alpha 1 PA 4.5 build, while all the other listed features can be found in the Stable PA 4.43 builds, too.
All the PA 4.5 Alpha 1 builds for Nexus devices can be found here.
For other Android devices you are advised to look at the legacy AOSPA project page here.
This step taken by PA team is one in the right direction and we can't wait until they manage to add multiple other new Android L-Style features in their future releases. Check the Paranoid Android Google+ page in order to stay informed on the latest added features and improvements.