One of our employees (thanks Sue!) recently alerted me to the fact that I was at 94% of my Gmail quota. Although we use Google Apps to host our business email, we've been able to get away with the free version since we're under 50 users.
To say that I'm an email fiend is putting things lightly. While I was at 94% capacity, nobody else among our 42 users was even close. The nearest 2nd place was at 13%, with few others above 2% or 3%.
I didn't want to upgrade to the larger account (and have to spend $50/year per user) just because I was approaching the limit. So instead, I used a program called FindBigMail, which charges $4.95 to analyze your Gmail account to find large mail items. Their tag line is "Pay $4.95 to save $50/year" and it works, because they're right!
There's also a great Lifehacker article on how to find large mail items, since Gmail doesn't let you sort by attachment size. Below is what the FindBigMail analysis looked like after it ran for about 30 minutes. I was able to forward the large offending emails to a backup account I own, and then delete them from both the regular and sent boxes to lower my usage.