1° Free Hot Spots ~ They are often not where you want them to be. Either advertised or you just try to connect to some hotel and it works.
2° Internet cafes ~ 1 Baht/min They are everywhere, haven't tried rural areas though...
3° (Some) places you stay ~ you pay 50 Baht, get receipt with username, password and 2 hours, which you can use through the day.
4° 3G: buy sim card for 50 Baht, load it with 350 Baht, insert it in your netbook (mine has 3G module built in, alternatively you need usb dongle). Metered on volume, good for checking email and light surfing only, should last for 5-6 hours. Or you can put sim card in your phone and have a local number.
5° 3G: got to phone shop, buy sim card for 50 Baht, choose a monthly plan, there are several, (I have chosen 100 hours for 350 Baht), unlimited for a month costs ~ 1000 Baht.
6° In many areas there are commercial wi-fi providers: you see a strong signal, connect, type http://22.214.171.124 and you will be presented with different plans: daily, weekly, monthly. (unlimited monthly ~ up to 2000 Baht)
3G connection (5° and 6°) is VERY slow, not for Skype or uploading pictures. (I am using AIS provider, have not tried others).
The rest is bearable, ~ 2 MB/min