Swapchat is a third party client for the photo messaging service Snapchat. The app, which is available for both Windows Phone 7 and 8, allows you to send and receive the expiring photo and video messages that are the keystone of the Snapchat service. The app includes almost all of the key features of the official Android and iOS apps, including the ability to add captions and annotations to photos, see when your friends have opened your messages, and the option to specify the expiration time period.

Swapchat may not have been the first Snapchat client for Windows Phone, but it is the most polished, offering a user experience that compares favourably to the official apps on other platforms, and is in line with Windows Phone's design principles.

Swapchat works by accessing the private Snapchat APIs used by the official Android and iOS apps (i.e. it's been reversed engineered). This method is in violation of Snapchat's terms of service (not unusual for third party apps that access a "private" API), which means it may be in danger of being blocked at a future date either by technical (e.g. changed API) or legal means. As the developer notes: 

Please keep in mind that this app is not supported by Snapchat and might break at any time. If it does, I'll do my best to fix it as soon as possible.

This use of private APIs is becoming increasingly common, with a number of recent high profile third party (unofficial) Windows Phone apps used to access popular services opting to take this route (e.g. Instance for Instagram).

In general we feel it's not an unreasonable method, especially in cases where there is no official Windows Phone app, but the these apps remain a legal grey area, and this is something users should be aware of before buying them. 
   

Windows Phone Store description:

Snapchat is the most popular picture chatting app in the world. It's a fun and addicting way to share pictures, it allows you to control how long your friends can see your messages before they disappear forever!

This app has almost all the features of the official ones:

• Take pictures, add captions, set a viewing time and send it to your friends!
• View your friends' photos (and even videos!), and see when they open your messages

What's new in Swapchat v1.2?

• Fixed crashes on Messages page
• Added ability to draw on photos
• Added ability to unblock friends
• Fixed incorrect aspect ratio and image rotations when taking pictures
• Front facing camera is now flipped horizontally
• Minor tweaks and bug fixes

Swapchat can be downloaded from the Windows Phone Store. A free trial is available with the full app costing £0.99 / $1.29 / €1.29