Are you the proud owner of a truck, or even a decent-size SUV? City dwellers always need help delivering furniture, especially on fine summer weekends. Advertise online, or hang out at yard and estate sales. (Hint: introduce yourself to the sellers so you don't look like you're casing the joint.) Bonus tip: when you charge your customers, be sure to account for the price of gas.
Throw a pantry raid
Lemonade stands are a Norman Rockwell fantasy. These days, even a 10 year old could tell you that street vending has a shitty profit margin, and that's before you invest thousands of dollars in permits and insurance. But if you're a good cook with a decent kitchen, you can squeeze lemons and make . . . money. Organize communal-cafeteria group-cooking sessions and charge a fee. To get diners you trust — and promote the hell out of the events — use Facebook or other social-networking groups. It's a win-win: you charge other people for the food you get to eat, and you help your culinarily challenged friends stock up on freezer-ready entrées for the week (which will probably save them money, too).
Be an auction hero
Economy be damned: if you have, or can make, anything remotely worth selling, chances are somebody wants it. Yard sales are cool if you need to unload a lot of stuff immediately (what, you need to pay off those loanshark debts?), but you're likely to get better money on the Internet. Try Etsy (etsy.com) if you're crafty, eBay if you're not, and Craigslist if you don't mind putting up with scads of sketchy, haggling, no-showing assholes.
Now, as for what to sell, start by checking the bottoms of grandma's old dishes for trademarks and find out what they're going for online. (We're operating on the assumption that grandma is cool with this.) Watch your fingers, as you might be surprised by your suddenly hot plates. If you have any electronic equipment old enough to qualify as "vintage" (hint: even the 1990 Macintosh Classic moldering in your dad's basement counts), dust it off, and head for the Flea at MIT (mitflea.com; they're held every third Sunday, April through October; the next one's on June 21). And if you saved your old Transformers and My Little Ponies, you're in luck, you sentimental bastard: a discontinued toy can go for many times its original price, especially if it's in good shape. Be sure to look for Web sites that cater to specific collectibles, too. You're likely to get a better price there than in the eBay slush pile, and browsing their classifieds will give you an idea of what people are looking for.
And hey, if you can draw, consider drawing on commission. The patronage system is alive and well in various dank corners of the Internet. (Some danker than others — you heard us, deviantart.com.) Practice drawing wide-eyed cartoon foxes in compromising positions, and try not to get sued by Disney.