This program is quite nifty, It can decompress .Zip and .7z .

What you can use it for:

-If you download a file from here directly to your DS then you can decompress the .Zip file it comes in!

-Not enough space on your card? Easy! Just select a file and press A to compress it to .7z! Then when you need the file back just select it and press A to decompress it!

-Also works on DSi

-But beware! This has a 1/99 chance of making hundreds of corrupted folders in the folder your file is in!