How to add (vertical) divider to a horizontal LinearLayout?

Try this, create a divider in the res/drawable folder:

vertical_divider_1.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android">    
    <size android:width="1dip" />
    <solid android:color="#666666" />    
</shape> 

And use the divider attribute in LinearLayout like this:

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="48dp"
    android:orientation="horizontal"
    android:divider="@drawable/vertical_divider_1"
    android:dividerPadding="12dip"
    android:showDividers="middle"
    android:background="#ffffff" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button" />
    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button" />

</LinearLayout>

Note: android:divider is only available in Android 3.0 (API level 11) or higher.

Update: pre-Honeycomb using AppCompat

If you are using the AppCompat library v7 you may want to use the LinearLayoutCompat view. Using this approach you can use drawable dividers on Android 2.1, 2.2 and 2.3.

Example code:

<android.support.v7.widget.LinearLayoutCompat
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        app:showDividers="middle"
        app:divider="@drawable/divider">

drawable/divider.xml: (divider with some padding on the top and bottom)

<?xml version="1.0" encoding="UTF-8"?>
<inset xmlns:android="http://schemas.android.com/apk/res/android"
        android:insetBottom="2dp"
        android:insetTop="2dp">
    <shape>
        <size android:width="1dp" />
        <solid android:color="#FFCCCCCC" />
    </shape>
</inset>

Very important note: The LinearLayoutCompat view does not extend LinearLayout and therefor you should not use the android:showDividers or android:divider properties but the custom ones: app:showDividers and app:divider. In code you should also use the LinearLayoutCompat.LayoutParams not the LinearLayout.LayoutParams!

use this for horizontal divider

<View
    android:layout_width="1dp"
    android:layout_height="match_parent"
    android:background="@color/honeycombish_blue" />

and this for vertical divider

<View
    android:layout_width="match_parent"
    android:layout_height="1dp"
    android:background="@color/honeycombish_blue" />

OR if you can use the LinearLayout divider, for horizontal divider

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" >
    <size android:height="1dp"/>
    <solid android:color="#f6f6f6"/>
</shape>

and in LinearLayout

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:divider="@drawable/divider"
    android:orientation="vertical"
    android:showDividers="middle" >

If you want to user vertical divider then in place of android:height="1dp" in shape use android:width="1dp"

Tip: Don't forget the android:showDividers item.

It is easy to add divider to layout, we don't need a separate view.

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:divider="?android:listDivider"
    android:dividerPadding="2.5dp"
    android:orientation="horizontal"
    android:showDividers="middle"
    android:weightSum="2" ></LinearLayout>

Above code make vertical divider for LinearLayout