When I was a kid, we nicknamed my father "The Condiment King" because he loves to douse everything he eats in some kind of sauce. Given that he has always loved eating and had more girth around his middle than is healthy, this is not a good thing. My parents fridge is always full of mustards, pickles, relishes, salad dressings and bottled sauces of various types.
Now I am all for a good condiment because I love food with flavour and think blandness is a culinary sin, but when it comes to your health, there are good, bad and ugly things you can choose to put on your food.
A lot of foods we often deem unhealthy or fattening, like white potatoes or pasta (whole grain, of course), are actually very nutritious as long as you avoid preparing them with high fat, high calorie ingredients like sour cream, mayonnaise, butter, whipping cream, etc.
Vinegar - There are so many varieties and I love the zing they add to food. Cider vinegar and rice vinegar are my favorites. Cider vinegar adds sweetness (I always add to chili, curries and tex-mex type dishes), and rice vinegar, which is subtle and versatile (I use it in all my Chinese, Japanese and Thai-inspired dishes). Balsamic and red wine vinegar are great in Italian dishes. Most vinegars (except Balsamic and seasoned (i.e. sweetened) vinegars are calorie free. Can't beat that!
Mustard - I love mustard and there are lots of types to choose from. Dijon is my favorite, but it tends to have quite a bit of sodium. Watch out for honey varieties as they tend to have quite a bit of sugar. You can easily make a homemade one with good quality dijon and a tiny bit of good quality honey. If you like the "French's" kind, which personally I can't stand - my parents call it "Yellow Paint", it does have redeeming nutritional value. It gets it's fluorescent colour from tumeric, the spice that makes curry yellow, and tumeric is ridiculously healthy. Researchers have found it has anti-inflammatory and anti-cancer properties. Mustard is great on sandwiches, of course, but also adds tons of flavour to marinades and sauces.
Salsa - Again, so many varieties available now, from mild to crazy hot. Great on potatoes, fish, meat, eggs, etc.
Citrus - I love the fresh flavour of citrus juice and zest and find that I really don't need to add salt when I use it.
Nut butters - I love making sauces with sunflower seed butter, pumpkin seed butter, sesame butter, tahini, and various nut butters. Just watch the amount you use because they are full of calories.
Chutney - Good quality ones are fabulous, but they are generally full of sugar, so use sparingly.
Look Out For:
Bottled sauces and dressings - Teriyaki sauces, stir-fry sauces especially, tend to be full of nothing but sugar and salt, and are not terribly flavourful. You can easily make your own. Salad dressings can also be full of sugar and salt so read labels.
Mayo - The real stuff is eggs and oil. Nuff said. If you must, get the low-fat stuff. I know this is a foodie no-no, but I actually prefer non-fat Miracle Whip. I just prefer the taste and texture to real mayo, which I can't stand.
Sour Cream - Low fat or fat-free if you must.
Butter - I use it sparingly for the occasional decadent dessert only.
Margarine - Only use 100% non-hydrogenated, and use sparingly, if you must, for toast, etc.
Jam - Try using only the all-fruit types with no added sugar. Just don't think jams count as a serving of fruit, it's still just sugar!
Oil - You really don't need a lot and most of the time it doesn't add a ton of flavour, just tons of fat and calories. Try toasted sesame oil for lots of Asian flavour, you only need a wee bit. Try making salad dressings with a 2:1 ratio of vinegar or citrus to oil.
I also recommend learning to appreciate the taste of foods, particularly fruits and veggies, without anything on them. Unfortunately, my father still refuses to eat broccoli without pouring something over top. Grrrr!