If you use an Apple/Mac, there is a built in dictionary that can also serve as a Japanese-English dictionary. Also on Macs there is a language input source shortcut. This means you can easily alternate between various language (English, Japanese, Chinese, French, Russian, etc.) settings when typing by holding down the command key and the space bar. First, you have to open the System Preferences. Second, select Language & Text. Third, select the languages you want to use.
Rikaichan allows you to mouse over words on a web page to get kanji readings and meanings (for Firefox 57 and prior).
If you have an iPhone or an iPod Kotoba or Japanese are great dictionary apps. Kotoba is free and Japanese is less than 2000円. You can find both through the Mac App Store.
Pop Jisyo is a handy online dictionary (last updated 2010).
Space ALC is another handy online dictionary with some amusing translations at times.
If you want to test your Japanese language skills try the Japanese Language Proficiency Test 日本語能力試験. It is offered twice a year in Japan and there are five different levels.
Learn Japanese Online is an online Japanese school. Take small-group classes or private lessons, and learn real Japanese from professional Japanese teachers.
Meguro Language Center offers a lot of free PDF worksheets for all levels. They are very useful if you are preparing for the JLPT.
NHK News Web Easy Offers NHK news articles in easy Japanese, with readings for kanji and audio playback.