This is more a list for my own reference, but it might be useful to others, feel free to make suggestions as well and I’ll update it.
I use Anki. It’s available on Linux/Mac/FreeBSD/Windows/Android/iPhone/maemo, so there’s no excuses here. Also if you set up an account you can sync between different machines or phones.
You can also download pre-made decks from within Anki. I recommend “T’s Spanish Deck”. It seems the most intelligently constructed to me.
Verb Conjugation Charts:
Verb Conjugation Practice:
Argentine Spanish (Castellano) Slang Dictionary:
Someone should make an Anki deck for this and make it public…
Good Spanish Dictionary:
http://rae.es/rae.html - If you input a word then click the “Conjugar” button on the translation page, you can see a chart for the verb conjugations. This site is also great once you know some spanish for getting a better idea or the connotations and nuances of words as well as just get translations in Spanish.