OLED is a new display technology that can make thin, efficient and bright displays. OLEDs are currently used in mobile devices (such as smartphone and digital cameras) and are starting to appear in TVs as well.
The actual OLED materials are transparent, and indeed it's possible to fabricate transparent OLEDs (TOLEDs). There are actually some products on the market that already use such displays, although currently the largest available transparent OLED display is only 2.4" in size. OLEDs can also be used to create lighting panels, and transparent OLED lighting panels are possible and already shipping in small quantities.
Transparent displays can have interesting applications - displays embedded windows (for example in car windshields), OLED TVs that will be invisible when not used or wearable displays. The usability of transparent displays is limited, and we're still waiting for companies to find some real applications for those displays.
Transparent OLED displays on the market
While Samsung intended to launch the world's first TOLED device (the IceTouch media player) back in 2010, they never did so, and it seems that Samsung isn't focused on transparent OLEDs currently. Other companies however have since taken up the lead in this market, and currently we know of two producers that make TOLEDs: Futaba and Neoview Kolon. These are small (up to 2.4" in size) Passive-Matrix display made in low volume.
The first real transaprent OLED product on the market was Lenovo's S-800, a feature phone that uses Futaba's 2.4" QVGA TOLED panel. The second product on the market is the Indonesian Nexian Glaze M-9090 (that has a Neoview-Kolon made a 2.4" panel). 4D Systems, a display supplier, is now shipping 2" 160x128 transparent OLEDs. In April 2013 we posted a hands-on review with this panel.
Larger TOLED prototypes
Several companies, including Samsung and LG, are working towards larger transparent AMOLED displays. In past years Samsung unveiled several prototypes, including a 14" laptop with a transparent screen...
Transparent OLED Lighting
As we said before, OLED technology can be used to make lighting panels - these are thin, area-lit and efficient lighting panels, that can be made transparent. Unlike displays, transparent OLED lighting panels have more immediate applications (embedded in windows, for example).
The Fraunhofer Institute in Germany are shipping transparent OLED samples. They do not plan to ever mass produce these panels. In April 2012 we posted our hands-on review with those panels.