CheapShark.com

Simple interface that works really well, easy to use. The website does not require any registration, and only asks for your email to send you price notifications.
The only downside is that it does not include that many websites. I also found that sometimes it doesn't detect price drops, whereas other websites do (namely, IsTherAnyDeal?).

[Edited by Douze, September 30 2020]
To clarify, i've only ever used CheapShark a couple of times, and have switched since then to IsThereAnyDeal which provides loads of options for when and which conditions should be matched for you to be notified about price drops. I'd recommend you do the same ;)

Worst interface of a website I have ever seen!