Where can find older versions of Apps? Is is possible to download older versions of Android applications? 

Is there some kind of online archive for all versions of Android apps? Like how you can find older versions of some Windows app.
Some alternative repositories, such as F-Droid, allow you to choose which version you want to download; all versions ever offered are in the repository.

For most other sites, including the Android Market, the latest version is all that is available.