Once you’ve signed up for an account, you can create IFTTT ‘recipes’ to check for actions and events on one online service (e.g. Google Reader, Dropbox, WordPress, Facebook etc.) and use them as an automatic trigger of a predetermined action in another (e.g. Gmail, Google Calendar, Tumblr etc.)
Example: To keep track of journal articles I should read, I monitor journal table of contents RSS feeds and e-mail interesting posts to myself for later download and consumption. I use my iPad, my phone, and occasionally my PC browser to access Google Reader, but struggle with how fiddly it is to e-mail myself on my mobile devices (with my filter-trigger keywords in the message body) whenever I find an article I want to read. I’m sure I’ve missed articles I ought to have read through setting my action criterion a bit too high, as a direct result of how annoying it is to e-mail myself articles using the various Google Reader interfaces on my mobile devices. Today I set up IFTTT to check for starred Google Reader feed items, and automatically do everything else beyond this that I find annoying. Perfect!
IFTTT will check for custom recipe triggers every 15 minutes, so it isn’t something you’d want to use for actions you require to be instantaneous, but it’s perfect for situations like the above. The services with which it is integrated are many and varied, and the possibilities therefore nearly limitless.
UPDATE 16/04/2014: I just came across this page and found that I had referenced the now defunct Google Reader. When Reader died I moved all of my RSS feeds across to feedly, which IFTTT supports with identical functionality. I also apply the same rule to twitter posts I favourite, meaning that I have a Gmail folder in which IFTTT aggregates all of the stuff I want to read from both feedly and twitter.