It's extraordinarily fun to have them say things they can't pronounce. Or say something stupid in English and then have one of the other accents (non-English, like Swedish or Chinese) say it.
Try 'abcdefghijklmnopqrstuvwxyz'. In English, it's funny enough, but in other languages/accents, it doesn't sound like nonsense, it actually sounds like another language. o_o
'pneumonoultramicroscopicsilicovolcanoconiosis' is good, too.
'Lopadotemachoselachogaleokranioleipsanodrimhypotrimmatosilphio-paraomelitokatakechymenokichlepikossyphophattoperisteralektryonop-tekephalliokigklopeleiolagoiosiraiobaphetraganopterygon' sounds like it would be fantastic, but it sounds like they skip over a lot of it. Or maybe they're just talking way too quickly.
And an easy way to get around the 'after a few voices you can't use it anymore' is to just clear the cookies from that site.