These resources help you do quick company look-ups and/or create lists based on criteria like 'give me all the companies in San Diego that have under 250 employees.'
These items are useful for market research, competitive intelligence and following what's going on with individual companies. The first three links serve well as starting points because they are the most broadly-useful databases that cover all kinds of international public and private companies. The rest of the items have more limited use but function very well in the domains they address, so may be helpful when further information is sought.