In more-or-less reverse alphabetical order:
Dar Williams: Part of the first coffeehouse I ever went to (along with Lucy Kaplansky; see below). Her music has gotten a bit more heavily produced and less singer-songwriterly over the years, but I still love songs like "When I Was a Boy" and "The Christians and the Pagans."
Bill Staines: Wonderful old-time folkie; incredibly polished performer.
Solas: Great Celtic band, with both traditional and original music (and willing to explore the darker side of the music scene - "Black Annis", from The Edge of Silence, is a creepily beautiful piece).
Carlos Santana: Back when I was on the Ustafish in Pearl Harbor, one of the junior officers found himself stuck on duty the day of his Santana tickets. I bought one off him for face value, and had a pretty good time. (It helped that it was an outdoor concert; even then, I really didn't go much for super-loud music.)
Pendragon: A Rhode Island-based Celtic group, who I saw once in a very sparsely attended show at the Bull Run in Shirley, Massachusetts. Haven't heard from them for a few years ... did they go and break up without telling me?
Robbie O'Connor: Another great Irish talent who's moved to the Boston area and enriched my life (and many others) as a result.
Katryna and Nerissa Nields: I never got to see the full Nields band together (described by Nerissa, IIRC, as "the two of us and three guys named Dave"); but I did get to see Katryna and Nerissa when the two of them were touring on their own. Well, Katryna was about six months pregnant at that point, but I don't think her daughter was making any major contributions to the act.
Natalie MacMaster: A tremendously talented Cape Breton fiddler, she's also a pretty good singer and dancer.
Tommy Makem: This is the guy who got me into Celtic music in the first place. My father had a couple of albums that Tommy did with Liam Clancy, and we would play them on the boombox in our cabin in the backwoods of Maine.
USAF Band of Liberty: The Air Force's New England regional band, they played the first of the Fourth of July concerts at Acton's NARA Park that I attended. (NARA is an old gravel pit, about a mile from my condo, that was transformed by the town into ballfields, a swimming hole, and an outdoor amphitheater.)
Christine Lavin: Funny and folky - and, if memory serves, someone who I first learned of from one of barbarakitten_t's alt.callahans postings. Who else does baton tricks in a Unitarian Universalist coffeehouse?
Lucy Kaplansky: She and Dar Williams did the first coffeehouse that I ever attended. Kind of bummed that she hasn't been out my way for a few years.
John Denver: The first concert I remember going to because I wanted to (rather than one that my parents wanted me to go to, or one that the school wanted me to go to) was a John Denver show when I was still in my teens.
Connie Kaldor: Definitely on the folky side of my eclectic musical spectrum, but with strong doses of humor as well. When I first heard her do Dot Dot Com, I literally fell out of my chair laughing.
Clam Chowder: From my days at the Hopkins Science Fiction Association and various Balticons. Glad to see they're still crabbing along.
The Chieftains: Long time traditional Irish band - saw them at Boston's Symphony Hall a few years back and they durn near brought the house down. Dunno why the website is down, though.
Blue Man Group: Outrageous show. Just mindblowingly weird.
The Boston Pops: I've never fought my way onto the Esplanade for their Fourth of July concerts, but I've been to several of their Symphony Hall gigs.
The Battlefield Band: Yet another great Celtic band, even if their lineup does change every time they go on tour. The first time I saw them, they closed out their first set with "Proud Mary" on the Highland pipes. Holy cow!
Altan: Last but not least, stunningly beautiful vocals (in both Gaelic and English) and touching lyrics (well, I assume the Gaelic lyrics are touching; I know many of the English ones are!). The Blue Idol includes both English and Gaelic versions of "The Pretty Young Girl", with Dolly Parton (!) joining Mairead Ni Mhaonaigh on the English version.