I had applied for a US B1/B2 visa last year to attend a conference. Since the visa process took very long I could not attend that particular conference. However, the visa was issued with a 1 year validity. There is another conference coming up soon within the time frame that my previous visa is still valid. Can I travel with the same visa? Is there any other documentation required ? (I am specifically asking because citizens of some countries that do not need a visa to travel to USA for toursim/conference etc), still need to fill an online form. Is there such a requirement for me in this case ?
Visa is not something that get guarantee to enter in a country. So also in case you went in the first conference, it is a lot better to have documentations.
In your case, you have a valid visa, but it was issued for a specific reason, and you gave then specific documents. So take with you (carry on luggage) similar documents, e.g. conference invitation, program, hotel booking, etc.
If you have similar documents, it should not be a problem: they will had issued the visa also with new documents.
I cannot tell you about financial situation or family connection. If you think that before you were at limit of acceptability, you may need to bring new documents [mainly if you were jobless, and now after one year they could have some doubts]
The problems arise if you do not have similar document, or if the reason of the travel is different. These cases are more difficult, and they may requires a new visa (or you should bring convincing documents).
Last point. The ESTA is only for people using Visa Waiver Program (VWP) [it is not just being citizen of a country, it depends also on the purposes]. If you have a valid visa, the visa data is used, and you do not need ESTA.