helpgeek

By default, the built-in Recent Posts widget in WordPress only allows you to 
display up to 15 recent posts max, even if you specify a larger number. This 
is hard-coded into the WordPress core so any modifications you make would have 
to be redone upon every WordPress update.
No longer!
The Recent Posts Widget Unlimited functions exactly as the built-in WordPress 
widget but without the 15-post restriction. You can now specify an unlimited 
number of recent posts to display without resorting to modifying the WordPress 
core.

illuminatus7

An easy to use Recent Posts Plugin. This plugin will allow you to display the recent posts in a sidebar only on a category page.
New Features Added
<ul>
<li>Specify the number of posts to display in the sidebar</li>
<li>Show/ hide excerpt</li>
<li>Specify the number of words to be displayed in the excerpt</li>
<li>Option to specify custom Read More text</li>
<li>The Read More text is also a link to the posts permalink</li>
</ul>

Categories Recent Posts Widget

This widget displays the recent posts on a category page for that category

Recent Posts Widget Unlimited vs Categories Recent Posts Widget