I just realized that my tourist visa expires today. It's a 60 day one, and I haven't yet been out to immigration to get the 30 day extension added to it.

Is it still possible to extend it and get the extra 30 days, or do I need to leave the country, pay the 500THB fine, and reenter again?

What's expired, the actual visa or your 60 day permission to stay stamp? A tourist visa usually expires 90 days from date of issue. You may be able to go to immigration tomorrow and apply for your 30 day extension.

The 60 day stamp expires today (well, about 10 minutes ago). It's a double entry tourist visa and this is the second entry.

I read a couple of other topics here about people in a similar situation, and both managed to get their extension. I'm going to immigration tomorrow to see if I can just pay the 500THB fine and get the 30-day extension applied.

You can get the extension as long as you don't leave it too long. If you go tomorrow you should be fine. You will of course have to pay the 500 over stay and the 1,900 extension fee.

I did the same a few months back, realised the visa had run out the day before! I went to immigration and still got the 30 day extenson but had to pay 500 baht overstay.

Get over there soon!