Tomek

Writes in the slidebar the numbers of your posts

Post-counter

Writes in the slidebar the numbers of your posts

horvath.agoston

Very simple, highly customizable counter widget for showing difference in 
days from a specified date and time. Allows HTML in title and body. Can 
count up and down. Supports multiple instances.
Originally developed as a baby age counter, but can be used for any counting 
job.
The options in the widget are:
<ul>
<li>
Title is the widget’s title. It can contain HTML; its value is simply embedded into the resulting HTML page.
</li>
<li>
Content is the widget content field. It uses the <a href="http://www.php.net/manual/en/dateinterval.format.php" rel="nofollow ugc">DateInterval::format</a> 
function to format the field contents. You can use any field specifiers from 
that function. Its contents are also embedded into the resulting HTML page, 
so over the %-tags, you are free to use any HTML you want.
</li>
<li>
Date specifies the date we should count difference from. Since this 
plugin simply calculates the difference between ‘now’ and the specified date 
(in 24-hour days), it can be used for counting down, for dates in the 
future, or counting up, for dates in the past.
</li>
</ul>
You can have multiple instances of this widget, of course.
For an example usage, see the screenshots section.

Count All

Very simple, highly customizable counter widget for showing difference in days from a specified date and time.

Post-counter vs Count All