As @kandy metioned, it is already possible to load certain blocks (although, that seems to have been build for the pagecache, not partial reloads), but I couldn't find a proper JS library to achieve this.
When the action is called, the target is reloaded without reloading the complete page.
This solution works well for us, allows us to create fully ajax-reloaded pages. For example: The cart page allows you to clear an item from your cart an it reloads the products table, totals, shipping costs and the cart header. When someone estimates their shipping the shipping block is reloaded and the totals, but the products table.
Something like this would be really handy if it were in the core and would greatly improve compatibility with modules.