An input group with a checkbox:

<%= input input => { text_field => ['username'] },
          prepend => { check_box => ['agreed'] }
%>
<div class="input-group">
    <span class="input-group-addon"><input name="agreed" type="checkbox" /></span>
    <input class="form-control" id="username" type="text" name="username" />
</div>

A large input group with a radio button prepended and a string appended:

<%= input large,
          prepend => { radio_button => ['yes'] },
          input => { text_field => ['username'] },
          append => '@'
%>
<div class="input-group input-group-lg">
    <span class="input-group-addon"><input name="yes" type="radio" /></span>
    <input class="form-control" id="username" type="text" name="username" />
    <span class="input-group-addon">@</span>
</div>
@

An input group with a button:

<%= input input => { text_field => ['username'] },
          append => { button => ['Click me!'] },
%>
<div class="input-group">
    <input class="form-control" id="username" type="text" name="username" />
    <span class="input-group-btn"><button class="btn btn-default" type="button">Click me!</button></span>
</div>

An input group with a button dropdown appended. Note that right is manually applied:

<%= input input  => { text_field => ['username'] },
          append => { buttongroup => [['The button', caret, right, items => [
                              ['Item 1', ['item1'] ],
                              ['Item 2', ['item2'] ],
                              [],
                              ['Item 3', ['item3'] ],
                          ] ] ]
                    }
%>
<div class="input-group">
    <input class="form-control" id="username" type="text" name="username" />
    <div class="input-group-btn">
        <button class="btn btn-default dropdown-toggle" type="button" data-toggle="dropdown">The button <span class="caret"></span>
        </button>
        <ul class="dropdown-menu dropdown-menu-right">
            <li><a class="menuitem" href="item1" tabindex="-1">Item 1</a></li>
            <li><a class="menuitem" href="item2" tabindex="-1">Item 2</a></li>
            <li class="divider"></li>
            <li><a class="menuitem" href="item3" tabindex="-1">Item 3</a></li>
        </ul>
    </div>
</div>

An input group with a split button dropdown prepended:

<%= input input   => { text_field => ['username'] },
          prepend => { buttongroup => [
                          buttons => [
                            ['Link 1', ['http://www.example.com/'] ],
                            [undef, caret, items => [
                                  ['Item 1', ['item1'] ],
                                  ['Item 2', ['item2'] ],
                                  [],
                                  ['Item 3', ['item3'] ],
                              ],
                           ],
                        ],
                     ],
                  },
%>
<div class="input-group">
    <div class="input-group-btn">
        <a class="btn btn-default" href="http://www.example.com/">Link 1</a>
        <div class="btn-group">
            <button class="btn btn-default dropdown-toggle" type="button" data-toggle="dropdown"><span class="caret"></span>
            </button>
            <ul class="dropdown-menu">
                <li><a class="menuitem" href="item1" tabindex="-1">Item 1</a></li>
                <li><a class="menuitem" href="item2" tabindex="-1">Item 2</a></li>
                <li class="divider"></li>
                <li><a class="menuitem" href="item3" tabindex="-1">Item 3</a></li>
            </ul>
        </div>
    </div>
    <input class="form-control" id="username" type="text" name="username" />
</div>