Sliding Tile Puzzles

Absolute Lines
Challenging online puzzle game on a checkerboard.
http://gameknot.com/lines.html

Boxcar Blockade
A sliding box puzzle game with hundreds of challenging computer-generated levels.
http://www.rhymezone.com/games/bb/

Cube Slide
Hexagonal variation on sliding tile puzzles. There must be two adjacent open spots to move a hexagon.
http://www.24hours7days.com/Hexagons/index.html

Dance Puzzle
Line drawings make these puzzles quite fiendish. You can reach the Hall of Fame if you're good enough.
http://www.tdbs.is.nl/scripts/interactief/game/puzzel/index_EN.asp

kibuR's Fifteen
Double sided sliding puzzle. You cannot slide individual squares. Instead, you move squares by sliding rows and columns of squares at the same time.
http://www.dotsphinx.com/games/rubik/fifteen/

Nick Baxter's Sliding Block Puzzles
Offers photos and a Java applet to play rare and collectable sliding block puzzle designs online.
http://www.puzzleworld.org/SlidingBlockPuzzles/

OmniSlide
Online slide puzzle which has no empty space. Instead, the pieces wrap around when the entire row or column is moved. Also available for download and placement on web pages.
http://www.sapello.com/slidepuzzle/

9 Piece Puzzle
Put together the Boston University logo (Java).
http://scv.bu.edu/~aarondf/java/puzzle9.html

Quazzle
Online mixup puzzle game in which the object is to slide the pieces of each image to form a complete picture in the least amount of moves. Also offers a free downloadable screensaver.
http://www.quazzle.com/

Sliding Piece Puzzles
Different sizes of pieces make these puzzles much more difficult than regular sliding puzzles.
http://www.pro.or.jp/~fuji/java/puzzle/slide/index-eng.html