Never is a site an alternative for good ol' reading, but here's a good site I found which offers the ability to place a 'word of the day' web thingy (what is called...:p) on the desktop. Pretty handy, and there's a lot more to gear it. Many probably know about it, but if someone doesn't here's a helpful link.

http://www.thefreedictionary.com/

another good vocabulary builder, along with reading anything you can get your hands on, is doing the daily nyt ['the times' if you're in the uk] crossword, till you're good enough at it to handle the sunday ones...

www.freerice.com

Just check out http://www.wordsmith.org/ They have a very good a word a day email program.