Here's 3 of mine:
1.) Autoresponder (of course). It's the way I touch base with my community, sell my products and promote affiliate goodies as well.
2.) A Feed Reader. In this case, feedly: your news. delivered. . Invaluable for staying abreast of trends.
3.) Advanced Google Search. Did you know Google offers a huge array of search operators that allow you to zero in on specific documents like FB likes, PDFs, images with creative commons licenses etc.?
How 'bout you?