Battle for Wesnoth is a great game.
I love Turn-Based Strategy more than any other genre, and I have to recommend a gem from the late 90's called M.A.X. from Interplay. Stupid story, dated graphics. Best. Gameplay. Ever.
At Gog.com, you can get it (and it's inferior sequel) for $2.99. It'll run in Boxer, and you can choose true TBS gameplay or an innovative 'simultaneous' mode, but I stick with the turns model. The sequel is Windows only, but seriously, the old DOS original is the one you want.