Phase 6: http://www.phase6.co.za/opencms/Homepage
This service has been around since 2002, and it's probably the most professional. It is basically an empty tool that lets you create your own content. Available for Mac OSX and Windows, but the Mac version is in Beta. The blurb says content includes English for ESL, Spanish, other languages and academic vocabulary and terminology (science, law, medicine, economics)
Their blog says that they have introduced "The “i-tunes of education” concept:
"As of June 2009, we’ve got the following (publishers) on board: Diesterweg Verlag, Klett Verlag, Cornelsen Verlag, Langenscheid Verlag, Springer Verlag, C.C.Buchner Verlag, Mentor Verlag, and Martin’s Little Helper."Anki: http://ichi2.net/anki
This service is free and Open Source. Very friendly video introduction. Available for Windows and Mac OSX, for jailbroken iPhones and the iPod Touch. This is primarily for writing up your own vocab, but it seems to be geared to content sharing communities, so for example teachers can push contents to students.
I've seen this recommended on Das Englisch Forum.
This service was developed by a father-daughter team, and it now seems to be used with PONS content. Besides serving Mac, Windows and Linus, it has a mobile app, but only for phones running Java MIDP 2.0. The focus seems to be on ready content that you can buy in small topical modules, geared to content at German schools. You can add your own content, as with the others, too. A USP seems to be the colorful skins you can put on your learning environment, which appeals to the kids.
Do any of you have experience using any of these, or other similar flashcard applications? Thanks for sharing tips!