David Quidnunc • Link
"The Cavalier Songs and Ballads of England from 1642 to 1684" edited by Charles Mackay is available online in at least two different formats.
The best way (I've seen) to browse through it is to go to the Project Gutenberg edition (scanned and proofed by David Price, online since September 1997):
Difficulties with this version are that it's all on one page, with all footnotes on the bottom.
Songs are on individual pages at the following website, but they're all mixed up with songs from other historical periods, so browsing is much more involved. But if you know the song you want, it's easier to read here and the footnotes are on the same, brief, individual web page:
Index page -- Songs of England
This site also has Mackay's introduction here: