You could find a lot looking back through this forum but I thought it would be good to have a new, consolidated thread. There are a several people just starting out with Kontakt or Komplete who would find this useful.
Please try to add some opinions and recommendations rather than just posting links.
Here are some to start the ball rolling.
Soni Musicae. - There are a couple very nice Harpsichords, they have different stops too so they have some versatility. I would recommend trying these first before buying a harpsichord library. The Orgue de Salon is a nice small pipe organ and the accordion is good too. There is also a Carillon which is in SF2 format but can be used in Kontakt if required.
SonicCouture - Apart from the Kazoos
the main thing here is the RMI Rocksichord. The Rocksichord was an electronic harpsichord, regarded by many as a ugly sounding piece of C*** but in its day was used by Rick Wakeman, Tony Banks, Terry Reily and John Lennon. I like it.
Analogue Drums - Big Mono is a excellent drum kit, the only reason not to add it to your collection is if you have the paid version with a few more features.
Hollow Sun -Part of the legacy of the late Stephen Howell are his free packs. These are mostly pads, very useable and with a fair amount of control over the sound. The Novachord is a stand out, but beware you may like it too much and have to go and buy the full version.