Miranda is free and open-source
Ranked in these QuestionsQuestion Ranking
Pro Requires no installation and is very lightweight
Very light on system resources and extremely fast, Miranda IM requires no installation and can be made to fit on a single floppy disk or USB drive.
Pro Insanely customisable
You can see all the plugins available for Miranda here.
Con Outdated XMPP Support
Missing OMEMO, Carbon Copy, HTTP-Upload ...