Google finally added an Labs option to Gmail I can actually use (and I've actually been waiting for it for a very long time). It's called "Refresh POP accounts" and it makes the link on top of your inbox called "Refresh" to also fetch any email you receive through any external email POP accounts you may have set up in Gmail (as well as Gmail emails).

This is very useful because Gmail automatic check only tries to fetch new emails for POP accounts every 30 minutes or so (it depends on the frequency of the emails you get through POP accounts - the more you receive, the smaller the time between the automatic checks).

To enable this new Gmail Labs feature, go to Settings > Labs and enable the "Refresh POP accounts" option.