I think current best estimates are that the most efficient charitable donations you can make save about one life per $2000 donated. If you're a decently but not extravagantly paid US software developer (I'm not, but the money is in the right ballpark) then maybe your salary is $100k/year; let's say you donate 5% of that before tax, or $5k/year; that's 2.5 lives saved per year. Most things you could do by volunteering full time probably do less good than that.

(Of course "lives saved" is an unsatisfactory metric; something like QALYs would be better. But the lives-saved figure is the one I happen to remember.)