Services like Foursquare, Gowalla (shutdown after being acquired by Facebook), Dodgeball (shutdown after being acquired by Google), Google Latitude (also shutdown by Google) are social community sites specifically designed for sharing checkins.
Why should you publish checkins? A few use cases:
If a high enough percentage of your friends use checkins, or view them, you can checkin to increase the chance of serendipitous meetups happening.
But to enable this use-case, checkins have to be really simple and unobtrusive, i.e. like the old Dodgeball txting UI was so simple that it made it very efficient to do so, so more people did so more often, thus enabling serendipity.
I'm entering a specific venue, I'd like to quickly/automatically capture the fact that I'm entering that venue (similarly for exiting) for personal logging / quantified self purposes.
Personal tracking could be done purely with private posts, however, friends-only checkin posts, without any expectation of anyone showing up, serve the purpose of also letting friends know where you are (and thus providing a feeling of being implicitly connected, even if mediated through such an indirect means).
I'm visiting a city other than my home town, and rather than try to figure out who I know may be in that town at the moment and compile far too many explicit messages or a long BCC email, I'd like to announce to my friends who happen to also be in that city that I've arrived there and will be around a bit in case they'd like to meet up.
I'm out at a coffee shop and want to quickly see if any friends are are nearby and might interested in getting brunch/lunch/dinner/drinks together.
finding people in a crowd
I've just arrived at a busy place/event, and I want to see if other people I think are coming have arrived yet so I know whether to look out for them.
awkward situation mitigation
why not to checkin
How to markup a checkin post: ...
What are best approaches to POSSEing checkins?
POSSE to Foursquare
POSSE to Facebook
Ryan Barrett currently manually POSSEs from his site to Facebook, manually picks the venue etc.
Can likely call Facebook API to get a list of venues at a geo location.
Could also create a Bridgy Publish feature that looked checkin post with a Facebook venue URL and then POSSE the checkin to FB.
For those not using POSSE yet.
PESOS from Foursquare
In rough launch/implementation order:
Barnaby Walters is using Taproot on waterpigs.co.uk to post notes with location data (including rev-geocoded address data) represented with an embedded h-card venue and in an embedded map, since 2013-01-24. Currently manually tagging specific check-ins with #checkin, but is working on adding richer venue information. Examples:
Some folks have experiments or prototypes of posts resembling or approximating checkins but are lacking key information in some way, e.g. explicitly marked up location information.
These posts are notes that show how a checkin could be presented in plain-text form, i.e. how a full checkin post could be POSSEd to Twitter, based on how Swarm cross-posts checkins to Twitter. Note how the Twitter copies have a permashortlink after their text content that links to the respective Swarm posts.
Past discussion sessions on indieweb checkins:
Pretty good notes in there that could be abstracted and captured in general form in new sections here.
More explorations/brainstorms than FAQ yet.
just a note with a location
Is a checkin actually a separate post type?
Or is it just a note with location information?
E.g. what are the user-expected/intentioned differences between:
More ideas than creations, some thoughts about how individual indieweb community folks want to do indieweb checkins.
Main article: venues.
I'd rather post a check-in on my own site, and have it use a venue URL that is *also* on my own site. Then at that indieweb venue URL, it can link to equivalents on Foursquare, OSM, FB, and whatever other centralized venue databases are created in the future. - Tantek 
Experience with Foursquare's "community generated/patrolled venues database" is that a few nitwits can pollute/damage the data quite badly, with deletions, overmergings etc. Sad to say, same thing happens with Wikipedia (excessive deletions, overmergings). Thus little hope that a "community hosted venues database" wouldn't have all the same problems, if not worse.
Just use hCard on indieweb venue URLs.
In general it is good to use multiple source of geolocation information in order to increase accuracy and decrease latency to provide this information when querying for and presenting (multi-factor prompts) nearby venues.
GPS is great. Except when you are inside a building - it doesn't work. Then you go and get on the Tube, and pop out the other side. It takes time for GPS to seek. You spend much more time waiting for GPS to work out where you are than you do just typing in the name of the bar or whatnot.
wifi-assisted location determination can do better than GPS when inside a building.
Last time(s) you checked in (or GPS was detected), you were moving in this direction at this pace, therefore you're likely at this location now.
The last n times you checked in at place A, you checked in at place B within the next thirty minutes.
You just checked in with friend F at location A, your friend F just checked in at location B, therefore you might be at location B as well.
plain text design
Background: "offline foursquare" projects
Use-case: wanting to checkin to a location without (good?) connectivity.
Facebook supports checkin posts.
Former Silo Examples
Silos that support checkins (checkin silos) that have either gone away (site-deaths), or dropped checkin as a feature.
Brightkite was a checkin silo.
Gowalla was a checkin silo.