Informal US terms for money amounts

"Nickel" is the commonly used term in the US for a five cent coin and based on its primary metal content.

"Penny" is commonly used for the one cent coin. This term is borrowed from British English.

"Quarter" is shortened from "quarter dollar".

You'll occasionally hear something like "solid quarter" as in "Do you have a solid quarter?" when someone wants to exchange smaller coins of an equivalent amount.

While a "bit" does equal 12.5 cents, when you hear the term it will most often be used in the phrase "two bits" which refers to a quarter. The word comes from the practice of cutting apart old coins called "Pieces of Eight".

"Dime" is the name that the ten cent coin actually carries. The name comes from the Old French "disme" which comes from the Latin "decimus". It's the only modern US coin I can think of which does not indicate its value. None of them use numerals, unfortunately.

A "bill" is sometimes used for "$100".

"Five spot" and "fiver" refer to a five dollar bill.

Sometimes currency is referred to by the person portrayed on the bill. This is more common with "Benjamin" or "Franklin" ($100), "Grant" ($50) and "Jackson" ($20).

Fiver = £5

Tenner = £10

Pony = £25

Monkey = £500

Also

Bob = 1 shilling (now five new pence)

Here are a couple more I've heard around:

• "big ones" ... slang for "thousands". Like "ten big ones" = $10,000
• "mil" or "mill" ... short for "millions". "5 mil" = $5,000,000

And here are a couple for Canada specifically. Not amounts per se, but slang for some of our coinage:

• loonie = the $1 Canadian coin
• twonie or toonie = the $2 Canadian coin

In the US, a “C-note” or “C-spot” refers to US$100.00.

Based on the British television shows that I’ve watched, a “quid” is £1.

With the growing popularity of Poker, a:

stack of high society

isn't just a "chip of the highest denomination" (usually $10000 in chips) anymore, but also 10000 actual dollars.

In the gambling community, you will find many other term to designate money amount.

Sawbuck, mentioned in JohnFx's answer is one.
But you also have:

• Dollar bet: a $100 bet
• Money: $500
• Nickel: $5 chip
• Quarter: $25 chip
• skin or skoon: one dollar