There are several SFG calculators available online where you enter the plant and it calculates how many you can plant per square. My understanding and experience is:

1 pepper per square

1 tomato plant per 2-3 squares

2 cucumber plants per square

Here is a FAQ on SFG spacing.

http://faq.gardenweb.com/discussions/2766684/what-are-recommended-plant-spacings

I think the spacing for corn has changed to 4 per square.

guess he can not be bothered to read the book which explain spacing clearly

it is simpler to read book Valerie since just looking up spacing does not work since you need improve soil too. plus the grid need to be used to keep space clear.

the books full useful info; can not be found online.

valerie I was being nice if he just wants to fail he is welcome to just use spacing. I got sick listen to stupid newbies who don't follow basic methods in any garden method around 2006. newbies need tough love to get over there stupidity by learning basics. the basic are a long way before you even get to spacing.

Well I'm a bit late to the party, but after reading that last comment I feel compelled to add my two sense Ryan to answer your question POLITELY, I plant my peppers one per square foot and stake them for support. I also plant my tomatoes one per sqaure foot but I keep them heavily pruned to no more than two stems per plant, removing all suckers and attached to a trellis. I had more tomatoes than I knew what to do with using that method. I keep my spinach at 9 per square regardless of the variety. And I plant then close to plants that grow quite large for shade. As for the melons and tzimbalo, I grow my melons up a trellis and support the fruit with nylon netting slings. Around one plant per 4 square feet. Last but not least, the cucumbers also need a place to climb and I put two plants per sqaure. I hope this helped!

Another thought is to double up, plant things like lettuce or spinach around a tomato plant because they are ready to harvest before the tomato plant gets enormous.

I have a hard time getting tomatoes to work in my square foot gardens. I put 4 indeterminates on a trellis, one per square foot, and pruned to one stem - no tomatoes. Next year I put two, one in each corner and used cherry indeterminates (Gardeners Delight) and it produced some tomatoes but not as many as my 5 gallon buckets and much less than in my larger garden. The soil is just not deep enough above the hard clay. This is a 4x4 using 2x6s with about 4 inches of Mels Mix. I suppose a deeper bed in 2x10s or digging in compost under the beds would have worked better.

slowrider, IMO, if you know you have clay underneath, I would break it up first than do the raised box over it. Again this is just my experience, but I had better luck with some stuff when I either raised the height of bed at least to 12inches or worked the soil underneath prior to putting raised bed down. I actually no longer use raised beds as I prefer the results from double digging and mostly following the sqf spacing. My way is a lot more work though.