eBay
Since a lot of people use eBay, the prices you'll find are a pretty good indication. Use this search to get a listing of recent bids on a card. It searches under the Magic: The Gathering category.
Search All Magic: The Gathering:
Kevin's Page O'Goodies:
This Magic: The Gathering price guide page has been around a long time. You can view his card prices by set:
Price guides from Magic: The Gathering - Online Trading League
They've got prices for foil and non-foil cards, and physical and Magic Online (MTGO) cards. You can also view Magic prices by set. Or search through different fields, like average price, high, low, etc.
Blackborder.com Magic the Gathering Price Guide
This guide is updated daily and automatically. It uses card prices based on the three cheapest prices in their database for a particular card.