For those of us who are bound to follow a special diet, be it gluten/dairy/wheat/sugar free then Infinity Foods in the North Laines is a must.
It has held its own in Brighton long before we were educated about these things. It is run as a cooperative and supports many of the local farmers, egg producers and small local businesses. There really is every possible alternative food available and the staff are usually always knowledgable.
Sadly it's not cheap, but then organic/decent produce never is. It will be interesting to watch its future as supermarkets start to increase their 'free from' produce. Hopefully they will not drown them out.