Battle for Wesnoth (open source: wesnoth.org) is a game that I started playing over 4 years ago. I looked far and wide for strategy games that could hold my interest with serious replay value. Once I found Wesnoth I got sucked in and I all but quit playing other games for quite some time, and I always keep coming back to Wesnoth, in a trend that players jokingly refer to as a "Wesbreak" - you just have to come back after a few months or a year to see what is new!
It is simple yet, rich with both strategic and tactical depth. Lots (lots!) of user made content and it's own markup language with which to write your own. I started off thinking that I'd never play it multiplayer, but that is about all I do these days. There are some co-op multiplayer campaigns (a relatively recently introduced feature) that are tons of fun. And the 2v2 and 3v3 games can be epic and utterly gripping - provided you find serious folks to play with. Like any multiplayer experience you will find people you'd like to avoid...
It's certainly not a game for everyone, but if you are into wargames, fantasy battle games, hex based games, rpg battle games or like to see thriving open source games you should take a look.
Disclaimer: I am a (very minor!) contributor, so I am all kinds of biased.
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