Drupal's native URL-rewriting functionality is pretty groovy; with a stock install of Drupal you can simply write the custom, semantic/human-readable URL for any node when creating/editing it!

However, things can get a little messy if you plan on writing URLs each time you add new nodes to your site; its makes sense to apply rules to how the URLs are written; such as making all blog entries follow a pattern like 'site.com/blog/[date]/[nodetitle]' - in case you're not familiar with it, there's a module which can do this really simply; its called Pathauto.