SebastianK

This Plugin is very easy to handle. Just install with a few clicks and set it up in your configs. 
Then include it in your sidebar e.g. with the following code:
< ?php spWeatherShow(); ? > (without spaces)

spWeather

This Plugin gets the weather from google.de/google.com and shows the weather now and a 4 days forecast with tiny pics

tiempocom

This plugin provides you the daily weather forecast for the locations you choose. 
With no configuration, you will be able to manage widgets and shortcodes with no hassle.
Features:
<ul>
<li>Ability to specify a different language for each widget/shortcode.</li>
<li>Locations all over the world.</li>
<li>Tons of options and configurations including: colors, layouts, formats, and many more.</li>
</ul>
Speed load impact:
<ul>
<li>Only a CSS file will be loaded, no JS on the front-end.</li>
<li>Images are loaded from the tiempo.com static file CDN server.</li>
<li>Internal file caching.</li>
</ul>
Important: In order to mitigate the site speed impact, please make sure to set write permissions to the <code>tiempocom/cache</code> directory. The plugin performs an intelligent expiry time management to ensure you have always the data up to date.
Supported languages:
<ul>
<li>English</li>
<li>Spanish (Spain)</li>
<li>German</li>
<li>Catalan</li>
<li>French</li>
<li>Italian</li>
<li>Portuguese</li>
<li>Euskera</li>
<li>Galician</li>
</ul>

Tiempo.com

Tiempo.com for WordPress. Allows to add weather widgets and shortcodes from the tiempo.com API.

spWeather vs Tiempo.com