I use this plugin: http://wordpresssupplies.com/wordpress-plugins/no-category-base/ Works well with URL rewriting.
The answer is as follows : nothing will completely solve the problem. If you use any hack or plugin and have second level categories like cars/brown you'll get 404 errors for the second level categories. It is ridiculous but I've tried everything and nothing has solved it if you go beyond level one.
You can't remove it, even if you manipulate the permalinks settings. The only way you can do that is to use a plugin. Or create your own. However, there is a better approach to this. Since removing the word 'category' may pose some problems (just like what others have suggested, the 404 and the like), why not use custom taxonomies for your project? Say you want to create a price category and you only want to display the word 'price' and no 'category' in your url. Create custom taxonomy for the term price and your permalink will display like so: http://dbpress.marcusblue.com/price/1-000-000/ Custom taxonomies work more like a tag than categories. It's not hierarchical like categories, but it is much flexible than categories especially if you have unlimited number of values to input that would prove to be long when you use categories. For instance, with price, if you use categories, you are limited to setting the price categories into ranges, like this: $1,000,000-$1,500,000 $500,000-$1,000,000 $100,000 -$500,000 Whereas with custom taxonomies, you may set just a single taxonomy 'price' and input the price for each post even to the nearest cent, like: $5,000.35 $456,999.87