There are many photoshop tutorials which help you to add a cool retro effect to your photos but,if you want a quick way to turn your photos into retro style then check out these time saving tools that let you add a retro effect to your photos without much effort.
vintageJS lets you add a vintage or retro look effect to your photo,although there are not many features available when compared to pixlr.
InstantRetro comes with social sharing options and you can quickly add several effects using their panel.
As the name suggests simpleretro is easy to use but you have to sign up for simpleretro to edit your photos,you can add a photo from your pc ,give your facebook photo a decent effect or take your picture via webcam.
If you want me to pick any one of the above i would go for pixlr-o-matic since it is easy to use and comes with a whole lot of cool effects.