Lately, we have seen an increasing interest in HTML5 and CSS3. The (not really recent) introduction of new features has been cautiously followed by web developers, and I don't know what's your feeling, however my impression is that the path to the new web is just full of pitfalls.
In order to test a web site for compatibility issues, I usually publish it in a not-known sub-dir and then display it with different browsers. I test it with Safari, Opera, FireFox, Chrome and Internet Explorer. In addition, I use services like Browsershots and IE NetRenderer. However, they usually take a lot of time (especially the first) and because we know that time is money, I recently decided to look for something else.
And I finally found a little software which made my day. The name is IETester and basically it is a browser which emulates Internet Explorer:
I used it a bit, and I must say it helped me a lot in finding little glitches and in the end it guaranteed a decent compatibility with Internet Explorer. Obviously, the software is not really complete, because it lacks all other browsers, but it significantly reduce the testing times. On top of it, I shouldn't remind you that IE is the most troubled browser in terms of compatibility.
Please have a look at it quickly, because I don't know how long it will stay free.