This is something that I bring up with clients a lot as it's a great tip to know. I want to put this list here to be honest so I have a quick reference to get here -- otherwise I end up Googling something like "date values in views and reports salesforce" to get the same list.
Essentially, I use things like this a lot for views. Let's say I want to create a view for contacts not touched in "the last 90 days". To do so, some folks create a view and do a "greater than or equal to" putting in a specific date - no need to do that at all. Relative date values will help.
Here's the full list: https://help.salesforce.com/apex/HTViewHelpDoc?id=custom_dates.htm
However, a snippet of that is here:
Here I can do things like this in a view and it keeps a "rolling 90 days" instead of a fixed 90 days. Hope this helps!