Online dictionary

[Kai Shi]

This is one of the best in my opinion.

jisho.org

Not only can you look up Japanese-English or English to Japanese but the large number of sample sentences can help you learn to put the word in context. Wonderful resource and super helpful!  

[Ryuondo]

I will endorse jisho.org as I have used it extensively. I have also found the Firefox Add-on Rikai-Chan and the program JLookUp to be quite helpful. Both were recommended to me by my upperclassmen. Both are free.

Rikai-Chan is a dictionary you can use by right clicking, turning it on, and then highlighting the Japanese you don't know.

And JLookUp is a dictionary program that sits on your desktop so it can be used when offline. It has a Japanese-English, English-Japanese, and Romaji-Kana functions. It does have dialect and archaic words so you have to be on the look out for them. It also has a Kanji Look Up function that allows you to build the kanji you're looking for out of radicals. It helps quite a bit when translating a kanji with an unknown reading on the fly. if anything, get it for that function alone.

Rikai-Chan

JLookUp Download page  

[Nana Shinu Ai]

I think that information will be quite useful.  

[Starry Starry Fright]

I used to use the Jim Breen Jdic in high school:

http://www.csse.monash.edu.au/~jwb/cgi-bin/wwwjdic.cgi?1C

It served its purpose.

Also, if you have an iPod/iPhone/iPad, I've found a few free apps that work nicely:

WordPowerLt
JLPTStdyFree
JpCard
Kotoba! (Based on Jim Breen)