I am a regular user of Amazon.com, and one thing that's always irked me is the company's use of excessively-long URLs. In fact, they are so long that back in 2002 Google increased the number of URL characters it was indexing just to accommodate them.
Now there's a rhyme and reason to this system, but try to explain that to the person who you just dumped a 150-character URL on in your IM conversation.
Instead of using a third-party link shortening service that might not work if it goes down, you should check out AmazonLinks. This small Firefox extension will automatically change any Amazon product URL into one that's far smaller than the original. It does this by replacing all the gobbledygook after the Amazon.com with an Amazon Standard Identification Number (ASIN) that still lives under Amazon's domain, but is about 80 characters shorter.
So this book's URL goes from:
Now if you want to take this one step further, you can shorten Amazon's URLs by hand using the company's not-yet-announced internal shortening system. This was unearthed by Go2.Me's Mike Koss back in April, and involves dropping the Amazon item number after http://amzn.com/. What I like about this Firefox extension, however, is that you don't even need to worry about digging that information up--it just does it for you.
One thing to note is that this extension is experimental, which means you need to be registered with Mozilla's downloads directory to download it.