1. Bliu bliu
Bliu bliu is a fledgling startup out of Lithuania that is showing a great deal of early promise. Basically it sources texts for you in your target language, and you read through them and click on each word for whether you know it or not. It tracks your words for you, and gives you other options for drilling down on words or exporting them for use in other programs.
Bliu bliu's advantages include a very easy to use interface, it works well at providing content appropriate to your reading level, and is looking at implementing more features. I look forward to more recordings of audio especially (and making some of mine own once I get that computer fixed).
I didn't know about tatoeba until quite recently. It's a project that aims to provide whole-sentence translations between languages. So, given an example sentence, users provide a translation of that sentence into whatever other languages they can. This is not mere 1-to-many though, as each sentence in whatever language exists as its own node, so users can then translate that sentence again. This creates an intricate web of corresponding sentences.
How to use Tatoeba? I have only just started on it, but it's a great place to do translation in a language you consider yourself high-advanced/fluent in, and it's a great place to find sentences in your language that you are learning. And recently bliu bliu seems to be sourcing tatoeba into its texts, which is great (the tatoeba database is CC-licensed)
I haven't used forvo that much myself, but forvo is basically a site collecting recordings of words of every language. This is a great tool if you want access to native speaker recordings of words individually. Combined with the above two tools, the ability to look up any word and find out how it's said should be excellent.