filk

Adventure Capitalists - WFH (these are friends of mine :)
Anne Prather - Hanrahan's Bar
Arrogant Worms - William Shakespeare's In My Cat
Blind Guardian - Lord of the Rings
Bob Kanefsky - The Eternal Flame (God Wrote in Lisp)
Cecilia Eng - Advice to Would-Be Heroes
Cynthia McQuillin - Apollo Lost
Doug Olsen - Fire in the Sky
Da Vinvi's Notebook - Title of the Song
Dr. Demento - Drunk Scottsman, Fishheads (you've probably heard this stuff already)
Escape Key - Ladies Don't Do Those Things
Flight of the Conchords - Business Time is probably the classic, or maybe If You're Into It
Frank Zappa - A Token of My Extreme (explicit lyrics)
A Halo Called Fred - Rock Me Amadeus (no really, and I like it better than the Falco original), (Hat)
Heather Alexander - King of Elfland's Daughter
Jonathan Coulton - First of May, Code Monkey
Julia Ecklar - Horse-tamer's Daughter, One Way to Go, Thoughts on Strange Visitors
Kathy Mar - Merlin
Leslie Fish - Queen Isabella
Mary Prankster - Mac & Cheese
MC Frontalot - Power User
Momus - Lucky Like St. Sebastian, The Seventh Wife of Henry VIII
The Monads - Let The Trolley Ride On Through (you'll want to be familiar with the trolley problem, a classic philosophical thought experiment)
Monty Python - Philosophers Song
Napolean XIV - They're Coming to Take Me Away
Ookla the Mok - My Secret Origin
S.J. Tucker - Eshkol, Firebird's Child, Valkyrie Daughter
Seanan McGuire - The Broken Heart of Fire
Tim Minchin - If I Didn't Have You
Tom Lehrer - Lobachevsky 
Tom Smith - I Want to be Peter Lorre (unless you've seen Babylon 5, in which case you might extra appreciate Five Years)
Urban Tapestry - Starsoul
Voltaire - Worf's Revenge
The Wet Spots - Large And In Charge (explicit lyrics)
The 21st Century Monads - The Transitive Property

Tom Smith - Falling Free
Julia Ecklar - The Phoenix, Everyman
Anne Passovoy - Harbors
Barry Childs-Heston - Lightsailor (the Black Book Band cover is easier to find)
Sassafras - Somebody Will
Frank Hayes - Little Fuzzy Animals
Diana Gallagher - A Reconsideration of Anatomical Docking Manoeuvres in a Zero Gravity Environment, The Moon Miners
Debra Sanders and Kathy Mar - Farewell Mother Russia
L A Filkharmonics - Farewell Terra
Leslie Fish - Hope Eyrie!!, The Light Ship, A Pict Song (Kipling)
Mitchell Clapp - Red Star Rising (hard to find, though; i've looked :()
Larry Warner - Ender's Game
Bill Sutton - Dust on My Feet
Fascinating Aida - Dogging
Jonathan Coulton - Still Alive

And heck, all of "Minus Ten and Counting" is on youtube now :)

Errata:
Fire In the Sky was by Jordin Kare

Seanan McGuire - The Black Death, Oh Michelle, Another Mad Science Love Song
Monzy - Kill Dash Nine
Three Weird Sisters - Dumb Dumb Dorothy
Jonathon Coulton - A Talk with George, Glasses
Deirdre Flint - The Shuffleboard Queens, Food, The Bridesmaid Dress Song
Bree Sharp - David Duchovny
The Billy Nayer Show - Christ
Lil' Nix - Off the Markov
Edelweiss - Bring Me Edelweiss
MC 900 Ft Jesus - If I Only Had a Brain