How to get current foreground activity context in android?

(Note: An official API was added in API 14: See this answer https://stackoverflow.com/a/29786451/119733)

DO NOT USE PREVIOUS (waqas716) answer.

You will have memory leak problem, because of the static reference to the activity. For more detail see the following link http://android-developers.blogspot.fr/2009/01/avoiding-memory-leaks.html

To avoid this, you should manage activities references. Add the name of the application in the manifest file:

<application
    android:name=".MyApp"
    ....
 </application>

Your application class :

  public class MyApp extends Application {
        public void onCreate() {
              super.onCreate();
        }

        private Activity mCurrentActivity = null;
        public Activity getCurrentActivity(){
              return mCurrentActivity;
        }
        public void setCurrentActivity(Activity mCurrentActivity){
              this.mCurrentActivity = mCurrentActivity;
        }
  }

Create a new Activity :

public class MyBaseActivity extends Activity {
    protected MyApp mMyApp;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mMyApp = (MyApp)this.getApplicationContext();
    }
    protected void onResume() {
        super.onResume();
        mMyApp.setCurrentActivity(this);
    }
    protected void onPause() {
        clearReferences();
        super.onPause();
    }
    protected void onDestroy() {        
        clearReferences();
        super.onDestroy();
    }

    private void clearReferences(){
        Activity currActivity = mMyApp.getCurrentActivity();
        if (this.equals(currActivity))
            mMyApp.setCurrentActivity(null);
    }
}

So, now instead of extending Activity class for your activities, just extend MyBaseActivity. Now, you can get your current activity from application or Activity context like that :

Activity currentActivity = ((MyApp)context.getApplicationContext()).getCurrentActivity();

I expand on the top of @gezdy's answer.

In every Activities, instead of having to "register" itself with Application with manual coding, we can make use of the following API since level 14, to help us achieve similar purpose with less manual coding.

public void registerActivityLifecycleCallbacks (Application.ActivityLifecycleCallbacks callback)

http://developer.android.com/reference/android/app/Application.html#registerActivityLifecycleCallbacks%28android.app.Application.ActivityLifecycleCallbacks%29

In Application.ActivityLifecycleCallbacks, you can get which Activity is "attached" to or "detached" to this Application.

However, this technique is only available since API level 14.

Update 3: There is an official api added for this, please use ActivityLifecycleCallbacks instead.