Do you duplicate products in the Magento admin interface? If so, you've noticed that Magento doesn't change the 'URL Key' field by default, which means every time you duplicate a product, you have to edit this field manually. If duplicating products is the main way you add new products to your site (and for many Magento users, it is), you're spending way too much of your time making this manual change, when Magento should be doing it for you. I've put together a quick userscript (so this requires Greasemonkey for Firefox or Chrome -> see Userscripts.org for more information on how to use a userscript) that checks if you're editing a Magento product's title, and if you are, it automatically updates the URL key. This solves the case of needing to update the URL key manually while duplicating products, and has a nice added bonus that any time you change a product's name, the URL key will be updated automatically.
The current version does some super-basic optimization on the URL key as well to make it a bit more Google-friendly, and I plan on updating it as time allows to enable automatic advanced search engine optimization of the URL key.