Ritual (21st & Valencia) used to be
the place to be, but they removed all the power plugs because laptop lurkers overran the place. They also replaced most of the tables in the back with a coffee roaster, so there aren't as many places to sit. It is still probably the epicenter of hipsters, coffee and nerds in the Mission. However, it may be easier to get work done elsewhere.
Mission Creek Cafe (20th & Valencia) has power, but the coffee is terrible.
SugarLump (24th near Bryant) also has power with slightly better coffee. Cool space.
Personally I am a big fan of Philz coffee (24th & Folsom) but I wouldn't want to work there.
If you want someplace quiet where you can get work done, Que Tal on 22nd & Guerrero is nice, sunny, decent coffee.
Cafe Revolution on 22nd between Valencia and Mission is a fun hangout. Lots of hackers (and non hackers) are there at night for music, coffee, and beer - but definitely not doing any work. Theoretically you could get some work done there during the day.