I take Boots own one-a-day multivit (primarily for calcium & B12, since I'm vegan), Floradix liquid iron (I've been mildly anaemic my entire life; it makes a noticable difference to my cycling ability!), and Evening Primrose Oil (stops me getting suicidally depressed once a month. I gather there's little to no evidence that it works as anything other than a placebo, but I Do Not Care, since without it, I spend 24 hours crying uncontrollably & eyeing up the kitchen knives, and with it, I don't. If it's a placebo, great, it still works.)
If I think I'm coming down with a cold I take Vit C 1g & zinc for a couple of days, which does seem to help.