I also spilled some salad seeds where my basket was when I was planting and it happened to be in the asparagus area. Asparagus will take at least two year to spread before giving any results so even though I wasn't planing on putting anything there it turned out to be for the good. This way I will harvest salad and asparagus will keep growing. It's a good combination.
And of course there are beans and peas which are good companions for most plants because they are natural fertilizers. They produce their own nitrogen - something that all plants need to grow properly. But do be careful with how much nitrogen you get in soil or some of your plants will be big and tall and leafy but not with much of fruits. For tomatoes it's better to be on a moderate side or you won't see whole lot of tomatoes later on.
And there are some combinations that really bad for each other - peas for example won't tolerate any of the onion family. There are many websites that have comprehensive lists of what's compatible with what plant so you can always google them. I know you can plant beans and corn and pumpkin - trinity of the farming world and they will grow wonderfully together. So keep experimenting. You never know what you might get :o)
One plant will give me enough seeds for all my needs. I have other plants that I use for cooking so it's not a big loss. The same goes for my arugula and salad plants. For now I can still pick them but probably come weekend I will take them all out and plant seeds for fresh set of crops. But for now they'll be blooming and I'll leave at least few of them to go ripe so I can gather seeds. It's not a good idea to use the same crops in one place so where I used to have salads I'll probably put something else. I already spread some salad seeds where I had spinach and radishes. That way I can rotate them better.
And I'm going to do the same with peas, beans, onions and squashes if they'll grow too many at one time.
That's one of the reason I wanted to use organic and heirloom varieties so I can gather up seeds and dry them for later use.