Here are the ones I use the most:
Stands for Financial Visualizations in case you were wondering, and is probably one of the most popular stock heat map tool websites out there. It's also one of the first ones I came across. Finviz doesn't only offer a heat map for stocks, but also Forex, commodities and more. One of the things I like about Finviz is the ease of use and the ability to switch between views, like a 3D view or bubble view. It's a great website if you're looking for you next trade.
Stock Charts; Market carpet
Though not as sophisticated as FinViz, Stock Charts; Market Carpet has some features that make me come back every time. The nice thing is that you can not only 'sort' your map by price, but also by RSI, Bollinger Band width, Aroon Oscillators, PPO and more! This is especially useful if you're looking to buy on dips (or short on spikes; overbought). Furthermore you can choose how you want the chart displayed, a Point and Figure (P&F) or the regular OHLC or mountain chart. Excellent website to do technical stock research, highly recommended!
Another one of the free stock heat map tool websites is Stock Mapper. The difference with this website compared to the latter mentioned is the filters you can apply. From the very active to the 52 week high and low. Sort it by ticker symbol, percentage chance, volume or market capital - all these great features will eventually help you find the right stock to trade. Another excellent feature is the ability to switch between economic regions such as Europe or Asia, this comes in handy if you also are on the hunt for stocks outside the US. Once you click on a particular symbol you get presented with outstanding charts and news items that pertain to the company you clicked on. Unfortunately no Forex or commodity options.
Another one to check out is from Market Watch (need to login in order to see the heat map). Although it looks suspiciously similar to the stock heat map from stockcharts.com, there are a few features that stand out above the rest. The first one is that it has stock market news in the right sidebar, providing you with an instant overview of what's going on in the world of stocks. When you click on a symbol you'll directly see the mentioned stock on the heat map, as well as a chart where the time frame can be adjusted on the spot, along with basic info like P/E ratio, Market cap, dividend ration etc. It has a very user friendly interface, which makes searching for your next stock pick a snap! The only downfall is that you need to register for a (free) account in order to use it, which is not really a 'downfall' actually, it just takes a few minutes of your time.
NASDAQ Dynamic Heat Map
At first, NASDAQ's version of the stock heat map tool may seem a little clunky (especially with the java applet in your face) but when you take a closer look it does have some features that the latter ones on this list don't have. First and foremost, the data comes directly from NASDAQ, which is always a plus since there's no intermediate server in between, but that's not why this stock screener is on this list. Its secret lies in the 'right click' when you hover over a symbol. It will present you with a list of options to choose from like; company news, analyst info, charts, holdings/insiders etc. In other words, more detailed information about the stock that you won't find with the other ones.
Although a stock heat map tool is very useful to find your next trade, it's of course not the only thing what you should base your trading decision on. Often enough it goes in conjunction with further technical and/or fundamental analysis, but at least you have a good start with this outstanding tool - and a good start if usually half the work. Personally before I make any sort of trade I always check the heat map first, the main reason isn't so much the overall info about the company, but the overall sector the company is operating in. I hope you found this article useful and if there's any other ones that should be added to the list; add a comment!