An amazing game of tactics brought to life by satisfyingly tactile playing pieces, truly one of the best games of recent years. Think chess crossed with Go, but with a nature theme - takes a few minutes to learn the rules, is instantly gratifying, but you can spend a lifetime becoming a master.
Probably best for ages 8+, as long as you're pitching against a player who matches your skill, you're guaranteed a great game. Comes with a handy carry-bag and it doesn't need a board, so it's the perfect travel game.
Check out an excellent review of Hive, all be it the pocket version (which only differs in the size of the pieces) here