This list below shows the resources I use when researching and valuing companies. These are all free resources. Please note that many of these websites have subscription services paid for on a monthly basis. I do not subscribe to any of these and simply use the free service. I am not paid for these referrals and all my recommendations are honest.

My Tools

LinkDescription
Investment ChecklistMy checklist covers the basic areas that need to be checked and are frequently overlooked by investors. It isn't exhaustive but it provides a good final check of an investment.
PE Ratio ToolThis was developed by myself as a valuation tool and wanted to share with others (it's free). It calculates the present value of future earnings based on input growth rates. The output is in the form of a PE ratio which the user can then use for valuation purposes.
How to find InvestmentsThis lists the most common places to check for investment ideas. With such a large universe of potential investments it's important to focus on the areas likely to produce good results.

External Resources

LinkDescription
ADVFNADVFN is a great website for financial information on companies. It gives an extensive set of summary statistics as well as detailed financial statements for a number of stock exchanges.
MorningstarMorningstar.co.uk is similar to ADVFN but has more financial statements for UK companies as well as an alerts service which I use mainly to get RNS alerts sent to my inbox. Unlike the LSE alerts, the RNS contents are actually delivered within the email.
Screener.coOffers a great screener with a free trial. I use this for all the stock screens posted on this site.
InvestEgateProvides the latest RNS updates for companies listed on the London Stock Exchange. Is great for searching for companies affected by news throughout the day.
ForexFactoryA great one stop shop for global economic data releases. Includes upcoming announcements as well as consensus forecasts. Useful for checking macro economic news.
Proactive InvestorsI like to check this site for information on director dealings in UK listed companies.
Info MineThe best website for commodities, including price histories and listed companies in the industry.
US Earnings Call TranscriptsGet transcripts for earnings calls that you can't attend. Very useful to go through as can contain supplementary information to the 10-Q
Yahoo Earnings CallsIf you prefer to listen to US earnings calls rather than read the transcripts, you can find them on Yahoo
Analyst Ratings NetworkGood place to easily find the analyst ratings of stocks and price targets
Yahoo NewsMy favourite source of stocks news, especially the sub categories:
UK Small Cap news
US Stocks News
Short InterestShows the US stocks with the highest short interest
Footnote ChangesA great new website that highlights the changes to footnotes in SEC filings from one filing to the next. Saves time reading through them all.
Investor PAKeep track of company announcements listed in Australia and New Zealand.
EstimizeCrowd sourced earnings estimates alongside analyst estimates, company estimates and historical results