I love Cowboy Magic detangler. It doesn't have precisely silicone in it (it does have another "-cone" in it though) and I find that for use <once a week it's fine. I only use it in Lacey's tail though, I find it kind of oils up her forelock/mane too much for my liking.
For her mane and forelock, my favorites are "Bee Nice" children's detangler from Great Clips or Dove conditioner for dry hair (one big blob of conditioner+spray bottle filled with water, shaken well). They work about the same and the conditioner option is much cheaper so I'm currently going with that.
Basically when you're choosing a haircare product for your horse, check the ingredients for silicone and/or "-cone". Any ingredients that end with "-cone" mean they are a silicone product. They may not be as bad as silicone itself but they shouldn't be regularly used. Anything "-cone" free is probably pretty safe for regular extended use.
However, of course, the best all time "product" is simply leaving the mane/tail alone. I only brush my girl's manes+tails about once a week, at most. As their manes get longer, I braid them up and during the winter, I braid and bag tails. That way, I can resist "messing" a bit more because they aren't getting visually dirty, you know? No "I must brush that tangle out!" or "ewwww! Dirt clod! Must remove."