http://www.moneyfactory.com/section.cfm/4 for pictures of US currency.
Not in the history on $2 bills is that they were often considred bad luck - possibly because they were often confused with $1 and you could be short changed. The "cure" was to tear off a little piece of the top right hand corner.
For lots of other neat stuff click :
http://www.ustreas.gov/topics/currency/index.html
I was having my car washed after getting it off the boat in Honolulu and proffered a $10 bill for the service. The manager said "I'll give you a free car wash if you can tell me how many cars are on the back of the bill." I identified the number correctly (4) and got a free wash!
http://www.treas.gov/education/faq/currency/denominations.html
 for answerss to questions on current denominations of currency - largest bills ever printed, present range of bills and that pesky $1 million bill.