Since I lived in Islington, most weekends I'd venture to Chapel Market, right off Upper Street near the N1 plaza. Take the Angel tube, cross Upper Street, and look for the street next to the M&S.
At Chapel Market, I'd buy fresh fruit and veg for the week. The market also had an assortment of typical market goods—sheets, cheap clothing, stalls selling electrical goods and even a cheese stand.
It's a fairly small market but worth a trip since the Islington area is kind of interesting. And there are more restaurants on Upper Street than anywhere else I've been in London. You're really spoiled for choice with all of the different cuisines available.
I also spent a lot of time at the N1 Plaza, right across from the market and the Sainsbury's supermarket. The N1 Plaza had a great movie theater, a Wagamama, a Borders, Next, Benetton, Gap, FCUK, a Lloyd's Bar, and at different times, an Italian or Chinese restaurant.