Luckily, such tools exist and are freely accessible on the web. No need to download software, you can access them via any browser.
For CSS there is http://www.csstidyonline.com/ which is an implementation of the CSSTidy library on SourceForge. Smiliar to this is also http://csscompressor.com/ and http://www.codebeautifier.com/ These not only compact the CSS (by removing redundant whitespace and comments) but it also intelligently consolidates and re-arranges styles to make them more concise. This leads to more efficient and cleaner code.
If all you want to do is minify / compress your code for production then you can try https://www.toptal.com/developers/cssminifier or http://www.cleancss.com/css-minify/. These optimise your CSS at the expense of being easily human readable. But if the only thing reading your CSS is a web browser, then readability is of little concern.
Google Closure Compiler
Whilst there are a lot of good minification software available one of the best is offered by the ubiquitous Google, under the rather obtuse name of Google Closure Compiler. This is available as a Java application but is also accessible via a RESTful API and, more importantly for us, online at http://closure-compiler.appspot.com/
To quote Google it,
UglifyJS and Others
Another very popular minifier library is UglifyJS.
But there are other minifiers available online, too, such as:
Visual Studio Extension
Sublime Text Plugins
If you use Sublime, then there are also many minifiers available, such as:
So there we have it - some great, free online tools that can simply and effectively compact your web assets to ensure your site loads as fast as possible.