sort posts by alphabetic plugin suggestion

Don’t know about a plugin but you can do that easily with query_posts()

paste this two functions in your theme’s functions.php file

function display_sort_links(){
    ?>
    <div class="sort_links">
        <ul>
            <li><a href="https://wordpress.stackexchange.com/questions/20830/?P_O=az">A - Z</a></li>
            <li><a href="?P_O=za">Z - A</a></li>
        </ul>
    </div>
    <?php
}

function display_posts_numbers(){
    ?>
    <div class="number_links">
        <ul>
            <li><a href="?P_P=20">20</a></li>
            <li><a href="?P_P=40">40</a></li>
            <li><a href="?P_P=60">60</a></li>
            <li><a href="?P_P=-1">All</a></li>
        </ul>
    </div>
    <?php
}

then paste this in the archive file just above you loop (if have_posts())

global $query_string;
parse_str( $query_string, $args );
//check for posts number per page
if (isset($_GET['P_P'])){
    $args['posts_per_page'] = (int)$_GET['P_P'];
}
//check for posts order a-z or z-a
if (isset($_GET['P_O'])){
    switch ($_GET['P_O'])){
        case "az":
            $args['orderby'] = 'title';
            $args['order'] = 'ASC';
            break;
        case "za":
            $args['orderby'] = 'title';
            $args['order'] = 'DESC' ;
            break;
    }
}

query_posts( $args );

and where ever you want the links to show just call the functions from before :

//for a-z z-a sort use:
<?php display_sort_links(); ?>

//and for numbers use:
<?php display_posts_numbers(); ?>