Perhaps not all of these are "obscure," but here are a few of my holiday favorites not yet mentioned:

Louis Armstrong (1953) - "'Zat You, Santa Claus?"
Jaymz Bee & The Royal Jelly Orchestra (1996) - "Sleigh Ride"
Cheech & Chong - "Santa Claus And His Old Lady"
The Eels (2000) - "Everything's Gonna Be Cool This Christmas"
Everything But The Girl (1994) - "25th December"
Arthur Fiedler (1959) - "Sleigh Ride (trad.)"
Babs Gonzales (1955) - "Be-Bop Santa Claus"
Big John Greer (1954) - "We Wanna See Santa Do The Mambo"
Vince Guaraldi Trio (1965) - "O Tannenbaum" (and the rest of "A Charlie Brown Christmas")
Jackson 5 (1970) - "Santa Claus Is Coming To Town"
Spike Jones (1947) - "Happy New Year"
The Kinks (1978) - "Father Christmas"
Eartha Kitt (1953) - "Santa Baby"
Tom Lehrer (1959) - "A Christmas Carol"
Aimee Mann & Michael Penn (1996) - "Christmastime"
Bob & Doug McKenzie (1982) - "12 Days Of Christmas"
Mitch Miller (1961) - "Must Be Santa"
Aaron Neville (1993) - "Please Come Home For Christmas"
The New Christy Minstrels (1966) - "We Need A Little Christmas"
NRBQ (1997) - "A Christmas Wish"
Bob Rivers Comedy Corp (1992) - "Wreck The Malls"
Adam Sandler (1996) - "Chanukah Song"
Jill Sobule (1995) - "Merry Christmas From The Family"
Nancy Wilson (1963) - "What Are You Doing New Year's Eve?"
"Weird Al" Yankovic (1986) - "Christmas At Ground Zero"

... one for each of the Beatles:
George Harrison (1974) - "Ding Dong, Ding Dong"
John Lennon (1971) - "Happy Xmas (War Is Over)"
Paul McCartney & Wings (1979) - "Wonderful Christmas Time"
Ringo Starr (1999) - "Come On Christmas, Christmas Come On"

... and so on, and so on. See this MetaFilter thread from last year for plenty more.