Chrome/Firefox: Sometimes, small apps add a feature that changes how you use a technology. Reply Now is one such extension that turns Gmail's timestamps from raw data to a relative measure of how long ago you got any email.
Basically, instead of seeing a time at the end of an email, you will see how many minutes, hours or days ago you received it, like "10 mins ago" or "3 days ago". It also colour-codes these, so it is green for the first 15 minutes, amber after 15 minutes, and red after an hour. These colours are a quick visual indication of which messages deserve your attention. The goal, of course, is to reply before the message becomes red.
It's tiny, it's free, and it's worth installing to see if it prompts you into addressing your inbox faster.