As to Plugins, I am partial to MoorMap, Alerter, AltInventory and a few others
Seriously though, Plugins will depend on what you want them for. You can browse through the downloads section to get an idea of what is available - don't be put off if they were posted a couple of years ago, the Lua API hasn't changed in over two years so many Plugins haven't needed any updates. I would recommend Lunarwater's Plugin Compendium to eliminate some of the simpler issues with downloading and installing them. There are two versions, one for Windows and one for Linux/Mac.
If you like roleplaying a musician, Songbook seems quite popular.
I don't use any "bar" plugins but SeqenceBars seems to have some attractive features and is still being maintained:
There are plugins for organizing Pet and Travel skills, just search the downloads section and you will find several.