Dr. Ross W Duffin at Case Western Reserve University has the Early Music Instrument Database
The Atlas of Plucked Instruments is a comprehensive, ancient to Modern, showcase that runs truly runs the gamut. Look here and be astonished at our diversity and commonalities.
Early Music Sources has a number of videos on instrumental and vocal music. Simply wonderful!
Songs are tricky! Up until the 19th Century people would collect either music or texts, but seldom both. It's either "frustrating" or "a fun challenge" depending on your point of view
The Traditional Ballad Index at Fresno State University. If they list a song, they know when it first appeared.
The Vaughn Williams Memorial Library of the English Folk Dance Society. Indexed collection of broadsides and music. If an original exists, its facsimile is here. Much more comprehensive than the Traditional Ballad Index, but a bit more difficult to use if you don't know in advance what you're after.
Mudcat Cafe is a broad crowdsourced source for "Traditional" songs. Come here if you want thirty variations of lyrics to Matty Groves (1611)
Mainly Norfolk is more highly curated, but more focused on recorded music
Professor Duffin wrote two books where he attempted to reconstruct the music of the Elizabethan Theater The Shakespeare Songbook and Some Other Note about the music in other Elizabethan comedies
The Session focuses on "Traditional Irish" so not great for Period music, but if you Do know a song is Period, here's where to find it in Modern notation, nearly always in G or D because "Irish D whistle"
Dame Constance Fairfax published a collection of catches and rounds, many from the Ravenscroft collections (scanned for printing 2-up)
Choral Public Domain Library choral scores throughout the history of choral music
International Music Score Project Library pretty much everything that Isn't choral
One of the better known collections from the 13th Century. Probably the most recorded collection of Medieval songs known, with many available editions. Mind that some of the lyrics are anti-Semetic, anti-Islamic, anti-Romani, etc etc etc, so use with care.
Oxford University's exhaustive database.
Cantigas de Santa Maria for Singers. All the lyrics and proper pronunciation
Gregory Blount of Isenfir (SCA) has a collection of links related to the Cantigas
Gaita Medieval Music has published a four-volume set of the Cantigas that comes highly recommended.
There are also free editions on IMSLP and ChoralWiki that might be useful to you.
The Pennsic Pile is a collection of music commonly done at balls. There are also back issues.
Al Cofrin has a fabulous collection of music online. You may know the name his dozens of credits in various Pennsic Piles.
I would never point you to Not the Pile, a collection of post Period and other disreputable songs
The late Layne Redmond was an amazing frame drummer and advocate. I cannot recommend her Frame Drum Intensive Study Series highly enough.
Carmine's Hafla Songs
A Very long list of rhythms can be found at the Middle Eastern Rhythms FAQ. But don't collect rhythms!
Glen Velez is a modern master of frame drumming.
Mickey Stewart runs the wonderful Bodhran Master Class YouTube channel.
Dame Amalie D'Anjou has a great list of resources on music, especially that of the Troubadours and Trouveres. She's also has a seemingly infinite collection of bowed instruments.
Mistress Kasha Alekseeva has compiled a number of Singable Translations of Period songs, quite useful for getting your feet wet. Read her essay on the challenges of translation.
Master Johann von Solothurn is the first Renaissance Guitar Nerd I ever found in the SCA. He wrote the delightful Come Along
THL Siri Toivosdottir is the sort of external music brain everyone with a dodgy memory needs. She has quite a bit of useful things online:
This section isn't very well categorized. I mostly drop links in here when I discover them in the hope I find out more.
Manuscripts of Italian Music Theory in Translation
Johannes Tinctoris is a 15th Century composer and theoretician. This website is working on translating his works.
Johann Joseph Fux was an Austrian theorist who wrote the standard work on the music of Palestrina. An English translation is in IMSLP
Gioseffo Zarlino wrote extensively. His earlier work on counterpoint is available in translation in a Norton edition.
Folias was an early form of Ground Bass.
Romanesca, or Watch the Cows! is another early ground bass
Open Music Theory, a free online text
The Venetian School, first identified in the music of Willaert, culminating in the music of Giovanni Gabrieli