I'm not sure if you can download for free if you click the download button after clicking ''watch this episode'', it doesn't work for me right now (I have the wrong computer so I can't check it out) but you can try ^^
Eyeshield 21 Episodes | Watch Eyeshield 21 Series - AniLinkz
On this site you must pay for the episodes and they don't have all episodes in avi, so you might not care for link at all...
Eyeshield 21 Direct Download
These ones are free, but once again; not all of them are avi. And scroll down to the MegaUpload-links those work now at least.
Eyeshield 21 | GalAnime - Free Download Anime and Manga
There are hella lots of links on this one, so I hope you can find at least one working link for each episode. ^^
Watch Eyeshield21 online , Eyeshield21 Streaming , Eyeshield21 Online For Free , Eyeshield21 anime stream, Watch Eyeshield21, Eyeshield21 Online, Eyeshield21 information,description,episodes and website links
And this one got some links farther down if you scroll down (like MegaUpload), but it's a troublesome thing when it comes to switching episode.
Watch Eyeshield 21 Episode 140 English Sub ~ Subbed Anime
I hope you don't need anymore sites... happy episode-hunting! ^^
Aaaand another link, just in case.
I hope the Direct download works for you.
Eyeshield 21 dvd-rip Ep 51-75
One more link...
Eyeshield 21 Episode 93 - Download
Now I'm done!