Second is Carbon Costume, another DIY costuming site with some innovative stuff going on. Some of their costumes are pretty pricey, though.
At the other end of the spectrum, Cosplay Tutorial covers everything from make-up to PVC, from wig-making to LEDs. NOT a quickie-costume site, but if you want to seriously get into cosplay it's an amazing resource.
Seconding the squee--and I hope Tom doesn't mind my taking this to Fandom Lounge.
(And mind if I seize a cue to grumble about another female costuming default, this one chiefly targeted at children: The Prettypoo Hello-Kittyoid Pink Version? It wouldn't bother me if it were one option among many, but just try finding commercially printed fabrics with a motif of fierce female pirates, soldiers, cowpokes, or whatever.)
Another exercise in frustration: have you ever looked for superheroine fabric? Superman, Batman, Spider-Man, Wolverine, and various Avengers have been all over the shelves; the Girl Power line by Camelot Fabrics--and I call attention to it as a blue-moon-rarity--lasted about fifteen minutes at my local JoAnn's:
And here's an example of pink used as just another color rather than a Prettypoo Token Girl flag; in all respects save color, it's identical to the brown version. (There was also a black colorway, but I was unable to find an image online.)
Best kid's costume I've ever seen was completely homemade - it consisted of a cardboard box painted blue, with inverted cottage cheese cartons attached (also painted blue). It was completed by a blue, long-sleeved shirt ending in blue mittens, and a blue cottage cheese container for a hat. Voila! Instant Lego Man. Probably cost less than $10.
About 10 years back, I took a bunch of the clip on flashing lights they sell for runners (and sell extra cheap this time of year for making trick or treating kids more visible) And attached then all over my shirt and the waistband of my pants.
Over that I wore a hooded cloak.
Walk up to someone, fling open the cloak and "I'm a flasher!".
At work each division is dressing up their department. Last year's winner was "PBS under President Romney" with binders full of women and an ad for the show "This Old Repossessed House". This year my department is down by two including the person who likes doing that sort of thing. But I think we're doing a theme about old broken technology. I'm going to put together a CD of computer themed filk songs (like the S-100 bus and "On the PC" trilogy). I thought about a software pirate costume, but not sure how to do that.
Unfortunately I don't have a regular pirate costume. Money is tight and it's hard to find things in my size. I did find a pair of pants, but the fabric is a bit thin and there are no pockets. I'd need a proper shirt, vest, and hat/bandana for a good pirate costume.
But I do have a bunch of thumb drives that I can wear on and I can make a sign that says "MS Edcel 2013, just $99 and 99% virus free" for my office (misspelling intentional).
If your presentation is going to include visual materials as well, don't forget this wonderful sketch from Amazon Women on the Moon--which references a previous generation's technology and media piracy: http://www.youtube.com/watch?v=7I5dVBezF9k
There's We're a Culture Not a Costume, the campaign started by Ohio University students to bring awareness about and fight against people dressing as racist stereotypes (not just blackface or Indian headdresses, but also "Redneck" or "Hillbilly" outfits that mock poor Southern whites).
It might well depend on the kind of dress you're wearing, though. If you're routinely a gingham-frock Mary Ann type of gal, a glamourpuss Gingeresque evening gown on Halloween will register as a costume (and vice versa.)