planting between peppers

Is there some plant or herb I can plant in between the foot & half spaced pepper plants?

Yes, Onions, Radishes, Beets, Carrots, Garlic.
I'm thinking possibly Parsley and Coriander (Cilantro).

also dont forget about marigold and calendula, both act as natural pest deterrents

Peppers like to touch so smaller types should be planted no more than 18" on center, not 18" apart from plant to plant. That would leave little light and space for anything in between. The other suggestions might work to the front of the peppers but peppers are generally not grown in the same season as cilantro or radishes and pulling garlic and onions just when your peppers are in it for the long haul would have to do some damage. Getting bushy plants that don't have to compete will make for larger peppers and prevent sunscald.

Do you have good luck growing peppers in the bay area?

So, out of my suggestions, that leaves carrots and beets.

Are you saying no cilantro in same season or no cilantro in same bed?

18" on center leaves 9" per side per pepper plant for root space. Considering small peppers such as tobasco, cayenne, seranno or jalapeños grow 24" or more, 9" of space for roots leaves no room for other planting. Larger varieties need 18"-24". Cilantro and arugula go to seed in late spring when the ground is finally warm enough to plant peppers. I often plant pepoers where these grew. Carrots and beets are planted after frost, in cool weather, several months before peppers so it would disrupt the root vegetables to plant peppers. You could grow and harvest greens (beets, mustard, turnips, kale, arugala, cilantro, parsley, etc.) and follow those plantings in warmer weather with peppers. Carrots would be hard to finish as they usually take many months to mature. While these vegetables can be grown together in a small space, they have different cultural requirements and time tables so won't do well mixed together.

Lisa has a point. The bay area is moist and blustery. How do peppers fare there?

This year I am using 12 tubs set 1.5 to 2 ft apart for 3 sweet peppers and 2 short vine melons or bush winter squash. Reason - I started too many peppers and all did well. Last few years I had peppers in buckets and froze a lot for fajitas. I tried melons in tubs last year but had tubs too close together and could not get to the melons. This year I decided to use just tubs for food crops and reworked the watering drip lines. I will report how they do - I can always remove some of the peppers if I have too many in a tub. In tubs I also have 16 tomatoes (6 cherry and 10 large), 4 cucumbers, 4 alpine strawberries, 16 salad crops/peas/beans, 8 strawberries, 3 blackberries, 3 yellow raspberries, 4 red raspberries, and 6 blueberries. Hoping this works. I want both peppers and small melons. I started the peppers, then tomatoes, then vine crops under grow lights, then put them in mini GH to harden off, then planted them in tubs - most had flowers already. Thought we were done with frosts.

Sweet basil.

I stuck a Contender Bush Green Bean plant between my Tabasco peppers. They're on 20" centers. The bush beans and peppers are doing great together.

The peppers and vine crops are doing well. I have been picking lots of ripe peppers for freezing. Not sure when melons are really ready - picked one that was supposedly ready but it was not. Tubs are 17 and 11 gallons on drip lines. Will do again.

Basil should be considered for sure.