Mostly Jitsi, Twinkle and QuteCom/Wengophone/WhateverTheyCallItToday. Plus various mobile clients, which tend not to live as long. On that point, though, Twinkle has been dead for years (which is a real shame, since it supported call encryption). I'm not sure what the current state of QuteCom development is (last I checked I couldn't get the development series to compile and it seemed like Linux was not high on their priority list -- an understandable but unfortunate decision).
It's been a while since I looked at Linphone; good to hear that it offers a SIP service -- I think the last time I looked, they were offering accounts but you had to register for them on the web site and set them up manually in the client, which is the awkward part.
YMMV, but in recent years it seems to me like the more interesting area has been the server and infrastructure space, like GNU SIP Witch.