Link to Hub / Hub Link
OtherHub
HubConnect
The three musketeers of hub interconnection apps. 
For ST <> HE, HubLink is an answer to the simple question of: how can I migrate away from ST?
In a two HE Hub scenario, the limited attribute exchange can become a show stopper.
A step up in capability for ST <> HE users is OtherHub, especially for people that plan on using ST for a while and need that bidirectional flow.
HubConnect targets the few of us with multiple hubs, especially more than two hubs. It works fine for two, but the extra install effort pays off as the hub count climbs. It implements a 'central' or 'coordinator' (server) role which may be ideal for a dedicated hub as the device count and automation complexities grow. Hubitat certainly encourages that growth by creating a highly extensible product. HubConnect is the only user extensible product of the three. As new devices or capabilities arrive, HubConnect allows users to create Custom Drivers for attributes not handled by the standard drivers without having to wait for the next release.
@bertabcd1234 gave an excellent overview and summary, really excellent, and my message is simply to reflect a little light into a few of the dimmer areas. 