IFTTT

 IFTTT  (IF This Then That) is a service for connecting apps and services with automation.

IndieWeb Examples
IndieWeb community members using IFTTT:

Jeremy Keith
uses IFTTT to POSSE his blog posts from adactio.com to Facebook. Example recipes for this (and more).

Aaron Parecki
uses IFTTT to PESOS his Withings weight measurements to his website by using the IFTTT "Maker" channel to convert the Withings measurements into Micropub requests.

fluffy
uses IFTTT to POSSE posts to Twitter, Tumblr, and (selectively) Facebook.

Chris Aldrich
uses IFTTT for both POSSE and PESOS with a variety of silos. See additional details in the posts listed below:
 * Using IFTTT to syndicate (PESOS) content from social services to WordPress using Micropub
 * Owning my RSVP’s from Meetup.com using IFTTT and Webhooks to a Micropub endpoint

Non-trivial latency
IFTTT has a non-trivial latency due to being limited to the once every 15 minutes polling they do.

No custom request headers
IFTTT HTTP requests to other sites have no way of being configured to send custom request headers.

SPOF
Because IFTTT is fairly popular (perhaps the most popular such service on the web) and in use by numerous sites large and small, it is particularly big SPOF (single point of failure). The lack of open source implementation (which you could move to if it existed by deploying your own) also adds to the fragility that you introduce when you depend on IFTTT.

Removal of app.net
In 2016-03, IFTTT removed the App.net channel, breaking POSSE flows in the process.

Removal of Pinboard
On 2016-03-27, IFTTT emailed users of the Pinboard integration saying it will be disabled seven days later. The creator of Pinboard wrote a detailed summary of why he won't agree to the new Pinboard developer terms of service: https://blog.pinboard.in/2016/03/my_heroic_and_lazy_stand_against_ifttt/

Removal of Twitter (portions)
Some time in 2016, IFTTT removed the functionality for several custom recipes that previously worked with Twitter. One of these, in particular, was the ability to auto-follow people who tweeted a particular hashtag.

Self-hosted (open source)

 * Node-RED, a lightweight and extensible Node.js application, uses regular files instead of a database, marketed for the Internet of Things but supports social services. Bundled in Raspberry Pi's Raspbian OS distribution
 * huginn, the most popular open source solution, written in Ruby, requires MySQL
 * Trigger Happy (GitHub repo), written in Python.
 * Beehive, written in Go.

Free Hosted

 * Microsoft Flow - as announced in 2016-04-29 Microsoft develops IFTTT rival called Flow for connecting apps and services

Paid Hosted

 * Zapier, a paid service with a limited free plan, mostly business-oriented, but supports personal services like Pinboard too
 * Botize, a paid service with a limited free plan