Those familiar with Salesforce record IDs, and more specifically their use within the Salesforce API will probably know that they come in both 15 and 18 character variations.
The 15 digit ID, as seen in the URL of any record within Salesforce, is a case sensitive ID, and can be confidently used internally within your org for managing objects.
If you intend to use, load or manipulate records with an external program, for example, exporting data via reports or the API, you will probably need to use the 18 digit ID, which includes three extra "check digits" and is not case sensitive. It is important to remember, that functions like Excel's VLOOKUP are not case sensitive, nor, by default are MySQL queries.
There are a number of functions and tools available to manually calculate the last three characters, and a really brilliant explainantion of how the digits are calculated, and their use can be found on the Astadia blog post 15 or 18 Character IDs in Salesforce.com.
This isn't an ideal solution though, which leaves you thinking that there must be a simpler way to get the 18 digit ID out of the front end of your organisation. And of course, you can, and once you know how, it's actually incredibly simple!
All you need to do is create a new custom formula field on your object, give it a relevant name, set it's type to "text" and in the formula box enter:
and that's it! If you're feeling particularly lazy, it's even in the righthand box of "Functions" and you can just double click it to enter.
Save the field, alter your field level security settings, and add it to the relevant page layouts and this field will display the 18 digit ID on your Salesforce record.
Now when you want to test, or work, with your Salesforce records, as they will have been saved in external applications and data exports, you can grab the 18 character ID straight off your page layout.