see the effect here in action: under pages ‘grid of posts’.

EDIT:
as this code can have its challenges, and is not always easy to apply, i would like to point to another approach with the same results:
Playing with columns – stacking posts in a grid.
END of EDIT

here, i am going  to present yet another solution: a flexible core structure to allow any number of columns with any number of rows, limited only by screen space, readability and fantasy…

a three column grid category template

the structure (for instance 3 columns, 4 rows, 12 or more posts):
1 5 9
2 6 10
3 7 11
4 8 12

the structure (for instance 3 columns, 4 rows, less than 12 posts, i.e. 7):
1 4 6
2 5 7
3

i’ll begin with a step by step introduction into the concept:

1. setting of variables
2. query_posts
3. calculate rows
4. loop 1 – the columns
5. the wordpress loop – the rows
6. page navigation

1. setting the variables:
$set_number_of_columns = 3; // enter numbers of columns here;
$set_number_of_rows = 3; // enter numbers of rows here
$set_showpost = $set_number_of_columns * $set_number_of_rows ;


2. query_posts:
global $query_string;
$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
query_posts($query_string.'&showposts='.$set_showpost.'&paged='.$paged);


3. calculate rows:
// make adjustment to get three equaly long columns even on last paged page
// get number of posts for this,
$num_of_posts = $wp_query->post_count;
// divide by $set_number_of_columns to get number per column,
// round up to next integer to make $ppc posts per column variable, or $set_number_of_rows whatever is smaller
$ppc = min(ceil($num_of_posts/$set_number_of_columns),$set_number_of_rows);
// calculates number of rows, i.e. showposts for the following query_posts and get_posts


4. loop 1 – the columns:
<?php for ( $col = 1; $col <= $set_number_of_columns; $col += 1) { ?>
<div class="col<?php echo $col; ?>">
<?php $row = 1;
$noffset = ($col -1) * $ppc + ($paged - 1) * $set_showpost ; //calculate offset parameter if paged ?>


5. wordpress loop – the rows and posts:
<?php $posts = get_posts($query_string.'&numberposts='.$ppc.'&offset='.$noffset); foreach ($posts as $post) : start_wp(); ?>
<div id="post-<?php the_ID(); ?> class="post row<?php echo $row; ?>">
<?php //output all post related data, i.e. date, title, content/excerpt, postmetadata, tags, etc. // ?>
</div> <!--closing .post div -->


6. page navigation:
<div class="navigation">
<div class="alignleft"><?php next_posts_link('&laquo; Older Entries') ?></div>
<div class="alignright"><?php previous_posts_link('Newer Entries &raquo;') ?></div>
</div>


here is the full code:

the naming of the css classes is:
class .col1 for the first column; class .col2 for the second column; and so forth.
same thing for the rows: class .row1 for the posts in first row, on so on.
going with this code is the css:
.col1, .col2 {float:left; width:49%; }

you can see the code in action: under pages ‘grid of posts’.
the effect ia achieved by giving each post a fixed height; and using a bit of javascript trickery.

lists or posts styled with alternating color backgrounds, for instance, can inprove the readability of blogs.

most themes and the standard wordpress installation are not offering this feature.
however, it is easy to implement, when you know how.

for example we could give alternating posts within the ‘wordpress loop’ a different extra class (‘odd’ or ‘even’) which can then be used with css.
to begin with, we introduce a counter ($c = 0;) before the loop, increment it inside the loop ($c++), and generate a class depending on the number of the counter by checking the modulus 2 of the number ($c % 2 – modulus is the remainder of the division):

<?php $c = 0 ; ?>
<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
<?php $c++;
if( $c % 2 == 0 ) $extra_class = ' even';
else $extra_class = ' odd'; ?>
<div class="post <?php echo $extra_class; ?>">
<!--typical stuff in the loop-->
</div>
<?php endwhile; ?>
<?php else : endif; ?>


the class ‘odd’ or ‘even’ can be used in the style.css to create different background colors, alignments, margins, paddings, font-styles – no limits.

the most basic application of this method with a counter is how to style the first post in the loop different (only showing the logic part of the code):
<?php $c++;
if( $c == 1 ) $extra_class = ' first';
else $extra_class = ''; ?>

to make it more interesting, the next example is to create three different styles and a special style for the last post on the page.
we can get the number of the last post with $wp_query->post_count .
The structure is the same, the main difference is in the ‘if, elseif, else’ logic:
<?php $c++;
if( $c == $wp_query->post_count ) $extra_class = ' last';
elseif( $c % 3 == 0 ) $extra_class = ' three';
elseif( $c % 3 == 2 ) $extra_class = ' two';
elseif( $c % 3 == 1 ) $extra_class = ' one';
?>

you can extend this method to create any number of cyclic classes.

and you can use it in any situation in your wordpress templates, where you loop through some output – lists, foreach, for, while, posts, comments, …

special thanks to @esmi who dared venturing into the realm of more than two alternating styles, and who inspired me to post this article.