One of the members at PSP 3D (butterballer360) has compiled instructions and a program to let you transfer files (up to 2 MB) with your friend(s) through WIFI. This was 'discovered' during an attempted hack of running external code by embedding files into images, and although the attempt failed, we figured some people could put to use the process we used to embed the files into the images.
Basically, you have to embed the file you want to transfer into a image on your PC, and then send the image to your friend. Next, your friend copies the image to his/her computer and uses the same program to decrypt/de-embed the file from the image. Good if you want to secretly transfer files with your PSP!
Of course, there have been homebrew applications which allow file transfer through WIFI, like PSpoof, so it's nothing new in that aspect, but this process will work with 2.01/2.5/2.6, since it is not a homebrew app.
Download the files
here and enjoy.