A few of my friends use it with their mares. I used it quite a bit with my gelding who got very stressed when the mares were in season (as in scared s***less rather than let me at them...)
Another vote for angus castus (chasteberry). A friend who is a herbalist rates it highly. Feedmark do it in powder form and you can buy it in liquid form too.
By far the least expensive way to do it is to buy from Cotswold herbs - it is http://www.cotsherb.co.uk/
About £6 a Kg as I remember. Lasted me for ages as we only use a teaspoon in each feed (the powder not the berries whole)