Half-celestials are the fourth template reviewed so far, and the first of the MM's three halvies. The template can be applied to any reasonably clever (gibbering mouthers and up) living creature that's not evil, turning it into a (native) outsider. Anything with RHD also has its number of skill points increased.
Their powers are varied, ranging from increased physical and mental abilities to wings and SLA's. In addition, half-celestials have a small number of random bonuses, such as immunity to disease and energy resistance.
Compared to half-fey, half-celestials have better skills and ability scores, slightly stronger immunities and resistances, but weaker flight and SLA's. These differences are large enough to warrant an extra point of LA, I'd say.