The Authoritative Minesweeper - World records at all 3 levels, a few tips and cheats, and many links to software and related pages.
Bomb Hunter - Commercial mobile-phone version of Minesweeper with new tools including probes and a radar.
Crazy Minesweeper - Windows shareware implementation of Minesweeper and a new variation in which new types of mines contribute 2 or 3 to the number seen in adjacent squares.
Hex Minesweeper - Variation on a hexagonal grid. Windows 95 shareware.
Hexatron - Variation of Minesweeper with hexagonal grid. Java applet.
Lady Di's mines - Java Minesweeper dedicated to Diana Spencer, Princess of Wales and her campaign to ban landmines. Includes high scores.
Mines-Perfect - An improved minesweeper-clone that generates guaranteed-to-be-solvable puzzles (never guess anymore) and has a Murphy's law option (guessing causes failure), more boards, sound, load, save, undo.
#1 MineSweeper - Java implementation with three levels of play, and an online hall of fame, which requires two games with a low time to emphasize skill over luck.
The Minesweeper Handbook - This handbook teaches how to play better Minesweeper, taking you from the fundamentals through to advanced concepts.
MineSweeper3D - Windows implementation of minesweeper on the surface of a 3D polyhedron, with challenging new tilings, and many boards to choose from. World records maintained online using encrypted submission. Built-in auto-solver. Demo version available.
Planet Minesweeper - Includes advice, cheats, rankings, downloads, and a forum. [English and French.]
Richard Kaye's Minesweeper Page - Papers about the computational complexity of Minesweeper, namely that the usual game is NP-complete and that an infinite variation is Turing-complete.
Truffle-Swine Keeper - A minesweeper clone that includes an algorithm to compute the probability that a mine is under a certain field.