Ok, here i go on my specialist subject...! The following are the seeds I have going spare, in varying quantities...
Things which take significant time/space/watering, long growing season:
Butternut squash, leeks, Brussels sprouts, Salsify, Broccoli purple sprouting, Purple cauliflower, kale, carrot
Reasonable amount of space needed, containers ok, better flavour than shops:
Courgette, Dwarf French bean, beetroot, tomatoes, peppers, chillies, pak choi, carrot, broad beans, peas,
Herbs and salads, space efficient and really make a difference: chamomile, Dill, sage, marjoram, thyme, Russian tarragon, Coriander, Chives, Rosemary, Nasturtium, Basil, Parsley, Spring onion, radish, rocket, mustard
I've put the groups in the wrong order really but if you have a small space e.g. a patio, balcony, window box, windowsill I would recommend the salad/herb group because it gives you the most back for just a bit of compost and watering. I see lots of good stuff for gardening on this scale on the poundland website so no big startup costs. I will be buying feed liquid/pellets from there for sure. You could use ice cream tubs for windowsill pots (poke holes in the bottom and use the lids as saucers), and inside-out compost bags for patio containers. I have also got three of the rectangular council recycling boxes in use as containers, and they would be deep enough for carrots, or anything surface-grown. Most other roots like parsnips need more depth but carrots can be done packed quite close together and picked young.
Inside out compost bags are good for potatoes, beans/peas, courgette. Turning out the black surface on the inside helps the soil to warm. For tomatoes and peppers, you would assume grow-bags are best but there's a trick to it - fluff them up so they're not solid. Also, check they're not more expensive than the equivalent weight of normal compost bags, which are a differently proportioned rectangle but you can cut holes in the side of just as simply.
Ask on free cycle for any excess seed potatoes, or just allow some to sprout that you've bought to eat - but not in the dark in the cupboard, put them on a bright but not direct sun windowsill. Maybe treat yourself to a pack of a good potato variety at the supermarket like Charlotte or King Edward. Put two or three on about 10cm of compost at the bottom of a bag, cover with the same depth, water and once the shoot is 20cm tall add more compost to not quite cover it. Keep going till you fill the bag. I'll come onto the next steps once we get that far 
or you could google it.
As well as the above I am doing soft fruit, chard, onions, garlic and shallots. Unlike potatoes you can't really do onion type veg from your spare shop bought ones. Growing garlic and shallots is probably quite good value compared to buying them because they keep well, if you have a cool dry place, but if you don't use them in cooking in the first place maybe not. Chard is good as a slightly more interesting veg than spinach but you can have too much - don't grow more than about five plants. Finally, in my massive essay on DIY food... Preserving. I am self sufficient in jam and chutney this year and a lot of my friends didn't need to buy jars for about a month either as I offloaded all that I couldn't fit in the house!! I did this by scrumping apples, plums and blackberries from the woods around my house.
Time for 
now...