Browser Based

Ajax
Java othello applet with a friendly interface and a very strong engine.
http://abulmo.club.fr/ajax-en.htm

GameSite 2000 Reversi
Online reversi with chat, customizable graphic interface, and play against live or internet opponents.
http://www.gamesite2000.com/reversi.htm

Interjeux - Othello/Reversi
An online playing version of the Othello/Reversi game.
http://www.interjeux.net/jeux/othello

Java Othello Openings
Large catalog of othello openings, viewable in a java applet.
http://www.othello.nl/content/anim/anim.html

Kelly's JavaScript Othello Game
Variant with game play options including one, two, and no player games.
http://www.posi.net/software/othello/

Kurnik
Offers real-time playing, game archives, and chat.
http://www.kurnik.org/intl/en/reversi/

Medusa
Has an interesting but intuitive user interface.
http://geocities.com/h_beasley/medusa.html

MSN Reversi
Multiplayer game play. Includes game overview and competitive tournaments.
http://zone.msn.com/en/reversi/

Neave's Webgames - Hexxagon
A variation of the game when the board is at an angle.
http://www.neave.com/webgames/hexxagon/

Online Reversi Games
A collection of online reversi games, including both single-player and multi-player versions.
http://boardgamecentral.com/reversi/online.html

Othello
Java applet with different levels of AI and an introduction to strategy.
http://www.site-constructor.com/othello/

Rev-Lux
An online Java program, with a winning percentage of over 90%. Adapts its thinking time to that of its opponent.
http://reversi.netfirms.com/