There is varying etiquette on sending people reminder emails. A reminder to attend an event is almost always alright, but a reminder to do something can often be considered brusque.
The norm when it comes to reminding someone to reply to your email, I've found that it can vary depending on where you're from. I remember being taken aback when told that it was quite kosher in American circles. A long time ago when I was applying for internships, not only did I send out emails with my CV and whatnot, but I also sent reminders a month later to people who hadn't gotten back to me the first time around.
Not only did this not offend the sentiments of any of the people I was writing to, but one such reminder was ultimately responsible for my spending a summer in North Carolina.
Email reminders serve a much broader function than just reminding people to reply, though. Sure, while some people really do get tonnes of email everyday where some things could get buried, many more of us are just chronic procrastinators. We get emails that we want to reply to, and intend to do so... only tomorrow. Tomorrow becomes the day after and then the week after and with the passage of time the awkwardness of penning a reply only gets worse, for not only do you need to reply, but you also need to come up with plausible excuses for not having gotten back sooner. The excuse here needs to be a compound one: one that explains why or how you forgot, and also explains how you remembered to reply to it *that* late.
Email reminders are perfect for this. They start with the supposition that you'd forgotten all about the original email until you received the reminder (and all you need to do is nod along,) and negates the need for explaining how you remembered.
And look! Now you've been prompt enough in replying when reminded. The first mail, the poor thing, it had gotten buried beneath fifty others.
Conscience cleared. Problem solved. Reply sent.
PS. Point of the post - People whom I've not gotten back to ought to send me reminders. Not because I forgot, but because I don't want to feel as sheepish. Just sayin'.