LoveSales - Find and save products you love, get alerted when they go on sale.
Domcomp - No-nonsense domain price comparisons, updated daily
TrackIf - Alerts you if the price of your favorite products drop
Pricepeek - Compare prices of the same product across all Amazon locales
Giftwhale - Giftwhale allows you to create personalised wish lists online.
The Mac Index - Compare Apple product prices in different countries